I intend to post more or less developed creature concepts here now and then. I expect some of them to already be considered by the developer, but it's difficult to evaluate which without more information, so here we go!

Watcher

Friedrich Nietzsche wrote:[...] when you gaze long into an abyss the abyss also gazes into you.

The Watcher is a poorly known denizen of Europa's sea characterized by an inquisitive (although, luckily, largely passive) behaviour hinting at an higher than average intelligence.
Watchers are usually found by the crew swimming alongside windows, peering through with an oversized, eerily human eye. They consistently focus on the closest visible active submariner, whether this one is located inside or outside the submarine, and sometime attempt to grasp or poke them with one of their many thin articulated arms (leading them to poke windows when their quarry is inside). Luckily these attempts inflict very little damage, if any to the crew or the submarine, and can easily be discouraged by the slightest damage inflicted on the slowly extending arm. The armoured, bulky body of a Watcher is remarkably resilient to harm, and the creature need to be severely wounded before it decides to flee. Of course, another way to get rid of a Watcher is simply to keep objects of interest out of its sight until it gets bored.
However, this passivity instantly disappears if their eye is targeted. The fragile orb is easily destroyed, and the blind beast then enter a state of mindless fury leading it to attack anything in reach: bashing its head against large objects (such as the submarine) and hysterically flailing its arms -which inflicts continuous damage to fleshy opponents in range. When hurt in this state, the Watcher ram in the direction from which the new wound originates in an attempt at vengeance; without such "input", the Watcher is able to localize and attack the large submarine through its other senses. Due to the aforementioned resilience, Watchers can then inflict sizeable damage to the submarine or the crew before they are stopped, unless one think to target the now bloody eye-socket, which lead to the brain of the creature -while such an assault still can't instantaneously kill the beast due to protective tissues being in place behind the eye, it is a lot faster than to wound the rest of the body, although such a level of precision against a mobile target usually requires divers.
Another way to easily defeat a Watcher is to take advantage of its blindness and simple reaction to attacks: it can be tricked into attacking another wildlife, or be permanently kept between two alternatively shooting divers.

The Watcher is intended to be a low-danger beast mostly intended to convey a mysterious and dreadful atmosphere when you cross its gaze in the darkness, though it can become an additional factor in crisis situation if poorly-handled.
I am not fixed on the intended size: a larger Watcher would be more intimidating and inflict heavier damages on the vessel, but a small one would open the possibility of one entering the gangways in case of breaches and playfully messing with circuits. Perhaps having both would be funny.

Inspirations (may help for appearance design):
- Colossal squids for their eerily human eyes associated with inhuman anatomy and size. - average octopi for their known intelligence and mischievousness, associated with inhuman anatomy.
- cetacean, for their intelligence.
- shrimps, for their many small arms.
I personally envision the Watcher as having the body of a cetacean/shrimp compromise, a large squid eye and a mix of tentacles and articulated arms as... well, arms. As usual, details are in the hands of the team artist.


Harlequins

The various species grouped under the umbrella name "Harlequins" are in league for the title of "less physical impressive animals of Europa". Bar few exceptions, these small bloated creatures are slow, gelatinous and barely cognizant. Of course, there is however a very good reason for the native fauna to avoid hurting them: Harlequins are living biochemical factories with a built-in self-destruct mechanism. Whenever one is mortally wounded, it reflexively pop, shredding its fragile flesh and engulfing its slayer in a cloud of dangerous compound (and making it very difficult for most predator to feed on the resulting gluey mess). Each species presents its own unique brand of chemical post-mortem revenge, and each is characterised by distinctive garish colouration and psychedelic skin patterns on its bulbous hump, allowing beforehand identification of the toxic agent by observant individuals.
Relying exclusively on the consequences of their death as a deterrent, Harlequins are unusually docile, to the point of being easy to pick up and move around by hand: their only reaction is to simulate agony (threatening imminent explosion) by making its hump bloat and pulsate, displaying light signals, and ineffectively flailing its weak limbs. When released, they immediately calm down. Due to this passivity, colonists have taken to capture, sometime breed, Harlequins for easy production of their corresponding chemicals: one just has to correctly identify the breed, corral them to prevent them wandering off, sedate them before "milking", and be careful during the extraction process so as not to "manually" trigger the detonation by accident.
Some examples of Harlequin gland produces:
- toxin: this straightforward substance hurts and repels Europan fauna (though giant monsters are too tough and stubborn to be incapacitated by it alone, obviously). While it hasn't evolved to hurt Earth-based organisms, most humans happen to be violently allergic to it, and are mildly hurt if they cross a cloud of this toxin without protective equipment. Rumour is that syringes of concentrated toxin would be quite the lethal and discreet weapon against colonists, but why would anyone do that?
- lure: using the principle of the biological "burglar alarm", this sticky substance slowly dissolve to release an odour that a lot of large European predators have associated with easy preys. Poor sprayed individuals can expect to be regularly visited by curious monsters for the minutes to come.
- explosive: most Harlequins use a weak detonation to spread around their payload; these species technically don't release any special additional substance, as the explosion provoked by especially volatile chemicals is punishing enough in its own. This variant is often used to make bombs; they can be put somewhere and then shot at a distance to produce the same result, but explosive Harlequins are among the most mobile and are liable to leave their post before one can get at a safe distance for the triggering.

It is important to note that the behavioural notes above applies to "solitary" Harlequins; other varieties are eusocial, like ants, and live in large colonies squeezed in drifting hives made of glued-together debris (such as bones, for example). In this case, various castes have different roles, and the castes tasked with protecting the hive care not about the value of their personal life: these unusually fast and active Harlequins rush down enemies of the hive and actively trigger their explosion in a kamikaze fashion. Approach Harlequin hives with caution.

I personally envision the Harlequins as looking like a cross between a tick and an octopus (for the bloated body aspect), with colour schemes and flesh texture reminiscent of sea slugs. Variants have not only different patterns, but their proportions may also vary, with more mobile species having longer legs for example.

I like how you described them. Even with a picture too. I don't mean to bring you down but I think it's unlikely that a developer will see this, it might happen but i don't know.

CookiesMilk wrote:I like how you described them. Even with a picture too. I don't mean to bring you down but I think it's unlikely that a developer will see this, it might happen but i don't know.

What are you talking about? Reg does check the collab threads.

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I'll see if i can throw a few concept sketches for these creatures in a bit, they both seem pretty interesting!

Yay!
I have some other ideas, but they are unfortunately less detailed for now...

Cookie-cutter

This one is very cheap: I just wanted to draw your attention to an IRL animal that would fit right in in European water with almost no other modification than a reskin to make it more alien-looking. I'm talking about the only parasitic shark, the cookie-cutter shark!
[youtube]https://www.youtube.com/watch?v=KCbny4l-sXY[/youtube]
(video here if the integration doesn't work)

A diminutive deep-sea fish with disproportionately large teeth which often feed by tearing off one bite-sized chunk of flesh from even vastly larger preys before swimming away with it, and has been known for attacking swimmers and the most vulnerable parts of submarines. Oh, and it occasionally swims in schools. What is there not to love?
It still is small and not persistent, so a Barotrauma equivalent wouldn't be lethal per se; more of an annoying pest, easy to mix up with harmless fish but liable to shave off a bit of your current HP or damage unprotected objects; a whole school, while still not lethal, would inflict more impressive injuries. However, the little bastards could feel threatening when you do need every pixel of this HP bar to go against some leviathan, and the creepy little devil just pop up and make the fight harder -or force you to cautiously circle it to avoid the bite. I see the species as straddling the line between "danger" and "atmospheric entity": one more reason not to go outside if you can avoid it, but not dangerous enough to really prevent you from doing so.
Of course, such an animal would as often target other animals, but true monsters wouldn't really be inconvenienced.
(By the way, the cookie-cutter idea tie in with a suggestion I vaguely intend to develop later: introducing "variant" skins of monsters, sometime affecting their stats, often being mostly cosmetic. In this case, the famous circular scars could appear on monsters. Having giant beasts bearing scars of encounter with unknown entities clearly able to go toes to toes with them is a way to increase tension without increasing difficulty -as a scar would merely diminish the health of the bearer, if having any effect at all!)


Ghost

The topic "Suggestions from a submariner" made mention of sonar, and sgarv thought about enemies liable to be provoked by sonar use. Well, here is an opposed idea linked to sonar-related adversary: an invisible boss creature that can however be easily found by sonar.
Perhaps it's the same color as the surrounding water, with some bioluminescence to emulate the faint glow of the local luminous bacteria. Perhaps it's transparent. It may or may not have faint outlines or occasional camouflage "fails" that observant players could detect. In both case, the beast would benefit from being eyeless, as eyes work by being opaque and would thus betray the animal's presence. It could be neat if the Ghost would be one of these monsters which only react to sonar, as it would be very difficult to fight without -this way you ensure it only attack submarines with a sporting chance. Perhaps the submarine's sonar disturbs its own?
The Ghost wouldn't be as dangerous if its behavior weren't adapted to stealth: while usually barely fast enough to keep up with the submarine, it is capable of regular and impressive bursts of speed, which it uses to escape attackers (perhaps with a flash of light so as to confuse its pursuers -and confirm to players it was hurt indeed while giving them a brief vision of its silhouette) and immediately after having damaged the submarine itself. Rather persistent, a hurt Ghost usually relocates in another location anywhere around the submarine rather than fleeing. As a result, a typical fight against a Ghost is a sonar-assisted hide and seek in multiple rounds, with divers trying to converge on locations indicated by the sonar crew before the Ghost catches its breath, punches the submarine and relocates.

I've little idea on how the guy would look like (and sure, it won't be "seen" very often if at all, but I still think that's important to determine). Apparently it's a predator of large, armored, visual-oriented and clumsy preys, which it dances around, avoiding their defences, while making them bleed to death through multiple wounds. Its method of attack is powerful enough to inflict worrying damage on a submarine section but cannot be made often enough that it can do without stealth. Perhaps it does really punch the submarine with a limb while grabbing its victim with another; perhaps the fact it only attacks just before moving is because it strikes with the powerful tail it also uses to swim.
In any case, the creature should have a relatively complex silhouette, so that brief visions by the players would result in "WTF is that?!?" rather than "oh, it's just a weird [insert animal here]".


EDIT1:
More love for corpses
wait that came out wrong

Corpses and blood streaks hinting at previous demises are a staple of the horror genre; so are zombies, whose blatant disregard of natural order loosens the protagonists' grasp on sanity by showing how little what they thought they knew about the world can be trusted (also, zombies are kinda directly dangerous, too).
With this in mind, I'm going to discuss some ideas that draw upon this macabre mindset.
Three notes before beginning:
First, one of the appeal of using imagery of death is to use the proximity of the characters with the (un)dead to show how at risk they are; however in the case of Barotrauma, it is unrealistic to expect massive amount of colonist bodies, so most of the suggestions will concern wildlife bodies, thus avoiding this proximity notion. I still consider them artistically interesting, for two reasons: first, the "bigger fish effect": if something unnatural and horrific can affect a Coelanth or an Endworm, players will be shaken by the realization that not even such terrors are immune to Europa's perils, and then what chances newcoming humans have? (On the other hand, one might want to lessen cruelty on locals to give the impression of a sea inexplicably united against humans; your call.) Two, mainly using beast corpses enforces a concept of the submarine as a "safe(r)" haven protected from the alien phenomenons of the sea around, where horrors can only enter if players let them in -thus horrifying things happening to creatures emphasizes the emotional importance of protecting the submarine. And makes violating this little sense of safety by throwing alien invaders inside anyway this much jubilating.
One of the advantage of talking about dead versions of existing beasts is that making placeholder models should be easy: just throw together the "damaged" versions of wildlife which have such model alterations, and tada! You have a "zombie-like" variation. Useful.
The last thing I wanted to mention before going is that while I spoke about zombies, I don't intend to suggest incorporating blatantly supernatural elements in Barotrauma. But phenomenon ambiguously supernatural is okay to me, because it keeps players on their toes while keeping the wilful suspension of disbelief functional.
So now, onto the ideas.

- deads and dyings: the simplest implementation of macabre, spawning creatures already wounded or dead. While of little interest on its own, it can be selectively used to emphasize some notions: perhaps in missions about monster eliminations, mangled lesser monsters found around can attest of the ferocity of the one hunted. On the other hand, perhaps it is the targeted one bearing visible scars to show its violent past.
- defilers: once players are dead, the worst is behind them; not so much if creatures (perhaps specifically designed for this role) devour bodies in horrifically messy way -and, from a gameplay perspective, only regurgitate a limited portion of their meal's equipment, badly damaged by gastric acids. This is how much Europa doesn't want you here! Perhaps something with an extensible gullet and a transparent stomach to unceremoniously gobble up fallen bodies, before inner jaws break their meal down in full view; perhaps some animal wrapping tentacles or membranes glistening with digestive acid around their prey.
- greater husks: okay, husks already exist and aren't really undead-looking, but their defiling of the human bodies is thematically relevant to the macabre subject. In fact, they're already so good from an atmospheric standpoint that little can be added to them. Still, there is the possibility of having "huskified" versions of other creatures, to show how potent the infectiousness is. Of course this is a boring gimmick on its own, but as long as it is supported by interesting innovations in the affected monsters, it could make the players anxiously speculate about the limits of the parasitic organism, which is good.

The next two ideas are about parasitically animated zombies, and I want to introduce to you my model organism to show you what I mean: Say hello to Phronima; this group of predatory crustacean feed upon eyeless soft-bodied animals (such as salp of jellyfish) and have the particularity of hollowing out the body of their favorite preys to make themselves a mobile HQ: grasping the cadaver with their claws while swimming, Phronima approach unwary preys to drag them inside. Young are raised inside as well, feeding on their walls of flesh and the kills of their mother until able to hunt by themselves.
Now, Phronima is a splendid monster inspiration (in fact, there are unsubstantiated rumors of this animal having inspired the xenomorphs), and there is two ways I imagine one could go about it:

- burster: a creature that looks very much like the corpse of another monster, but this is actually the "flesh suit" of a parasitic predator. It plays dead until the submarine is close enough, then attack, still appearing as a dying or undead monster. When the disguise is damaged enough, however (which is easier to do than killing a living equivalent), the parasite inside suddenly shreds its disguise in an explosion of decaying flesh and coagulated blood, and goes to town on the submarine with renewed vigor and perhaps new strategies. At that point I imagine the fight would not last much longer, since the highlight should be the "HOLY COW" moment of the emergence.
As a personal stab at appearance design, I'd be going at a "skeleton monster" look to maintain the undead confusion a tiny bit longer. If possible though, something that quickly is revealed not to feasibly be of the same family as its host in any way -for a fish parasite, perhaps an arthropod/frog thing. Additionally, I'd make it a faster and more frenetic monster than when inside its puppet, to surprise the players. The Stoplight loosejaw would make a neat head inspiration; the lower jaw of this fish is roofless to diminish water drag when projecting it ahead, and its throat act as a second jaw to gobble preys down when they're kept captive of the first one. This is both skeletal-looking and alien. This "no flesh to eliminate drag" could be a good in-game explanation for a skeleton monster: to make it faster and more agile, provided it also have fins to propulse itself.
IRL, skeleton shrimps are also ridiculously emaciated (and transparent for added freakishness). Meanwhile, the ludicrous pycnogonids manage to rock the look of a cross between a spider and a ribcage by dumping some of their internal organs into their legs. With all of that, there certainly is a way to make a bloated, obviously dead but still aggressive fish explodes into a freaky emaciated ghoul with Mantis arms which dances around the submarine with manic abandon, helped by its unfolded fins, during the brief window of time before it is inevitably blasted into oblivion by railgun, adding yet more blood into the water.

-maternity: the other side of the Phronima concept, about the maternal instinct. This time, there is no way to mistake the infested corpse for a live but mutilated animal: it is clearly animated by the swarm of diminutive parasites that uses it as their home, and mirror the crew of the submarine this way. The giant body is inert, more vehicle than monster, but the puppet-masters protrudes to form weapons or, for the smaller ones, try to break open the submarine and enter to install themselves in this new host.
Perhaps we could even find a caste of "propeller" creatures lining up the corpse, moving it around until destroyed. Some might even detach, throw themselves at the submarine, and struggle against the engine to keep it in range of their brethren.

Of course, the previous ideas can be combined in weird ways for additional !!FUN!!, as long as care is taken not to overplay it.

Oh neat I especially like the idea with phronimas, could add some new fun gameplay

I have to admit, these suggestions are very cool, in my opinion, especially with the ghost, looking forward to seeing it in the game. And these parasitic animals fit very well into the game itself. It adds mistery and adds to the plot, of being in the unknown seas of Europa.