These creatures seem to have been genetically (and literally) isolated for a long time, meaning they are likely genetically distinct enough to be given their own classification, I am going to put them all in the same genus as they share many similarities, I lack creativity, and I am not incredibly adept at classifications. So I'm calling the genus "formido" because that is the best google translate gave me for fear in latin.

This is the most common creature down in the caves. It appears to be some sort of very large, carnivorous, arthropod. This makes me think there is a lot of oxygen down there, as arthropods don't tend to be able to get that big without oxygen and their blood is red, meaning there is likely hemoglobin. I could also be completely wrong and these things are completely new, with a common ancestor being very far back, even compared with everything else. They have very bad eyesight, or just don't react to stimuli as much as other animals. This would be supported by the fact that their hearing sometimes seems terrible, and they generally react only to noises very close to them. They could also just value hearing less because echoes would mislead them, although they do seem to vocalize, which likely means they can communicate at least a little bit.

The dev has a name for these guys: Nope-Flies. I should have expected that, but you can find it in the emojis in the points shop. I'm going to keep the scientific name unless I find out he has one.

Small worms with sharp teeth. They also seem to be mostly blind as well as deaf, since they generally just wander around unless they get really close to you. They are able to climb surfaces just as fast as on the ground, which is to say not very fast. They do seem to be the same species as the flyers, but they could also be in a symbiotic relationship. I am leaning towards same species, so the scientific name is probably not valid.

These are large and insect-like. They have a long proboscis that they use to stab victims. They seem to mature in cocoons, which makes me think these are all the same species, just at different stages of maturity. They can climb most surfaces and will drop onto victims when given the chance.

This appears to be the final stage of maturity for all of the previous creatures mentioned. Here is my theory on the lifecycle:
1. Released as a small flying creature
2. Incubates in a flyer until reaching next stage of maturity
3. Released from flyer as worm
4. Matures a bit more and climbs out of its previous exoskeleton as flyer
5. Matures more, and then groups up with other flyers to create a cocoon
6. Exits cocoon as an insectoid
7. Fully grown and ready to reproduce (presumably after growing a bit)
This is a theory based off of my observations in-game. In combat the big insectoid releases small flying creatures which kind of look like flyers, which is where this came from. If this theory is correct, the previous creatures no longer get a scientific name (which is sad) and I think they should be grouped up into Formido Magnus Insectoidus, as they all fit that description pretty well.
All of these creatures don't seem to respond well to stimuli, even pain.
I looked at my trading cards and realised the dev named some things. Which I should have expected, given that they get to create all the lore. They're called Macrocheirants. I'm going to keep my scientific names for right now unless I find out he has one.

As stated by a commenter, the size difference is massive between the macrocheirant and the nope-flies, as well as the smaller spiders, there is also the fact that my suggested life-cycle is very convoluted. I would like to note that with 3 gestation periods I don't think it would be unreasonable for this amount of growth, as well as real life examples of this sort of size disparity between adults and children, such as pandas. I think that the worms and nope-flies are absolutely the same species and until further evidence is gathered will continue to think so, however we need more evidence before coming to full conclusions with the others. It seems reasonable that nope-flies would sort of mirror cicadas, emerging from the worms or perhaps making cocoons, maybe the ones seen on the wall. I find the proposition that the flies released by the macrocheirant to not be separate organisms but hunting aids extremely interesting, and given the slow movement of the macrocheirant may be worth the energy expenditure.

In Security Breach one of the tasks asks you to destroy the worms' nest, there are large amounts of all of the creatures here. This may give further credence to my theory, or show that this company either knows just as much as we do and is making the assumption that they are all closely related or doesn't care enough to do any research on them. To be honest, I am leaning toward the latter, they refer to them as contamination, destroy them with no regard for the biosphere, and the expedition seems to be largely motivated by the blue mineral, with some minor experiments somehow involving high amounts of radiation. Murphy should join a union because these are completely unacceptable working conditions, the dosages of radiation are enough to kill you in minutes, and in one level, less than 10 seconds in water, which should provide some shielding.

I have very little observations of these creatures. The move quickly at their victim, pounce, and then retreat. They don't seem to be the same species as the others in any way.
They spawn from the backs of the gas belching organism. From the dialogue in Security Breach they may be the same species, but as stated above I don't necessarily trust the company on this one. Co-habitation does not necessarily mean they are the same species, we only have confirmation of direct interaction between the macrocheirant and its horde and the nope-flies and the worms, which could both be examples of symbiosis rather than reproduction. Upon further observation "Scratching Fear" is fitting, they are extremely dangerous, especially in hordes, and make fairly creepy scratching noises similar to the spiders.

These seem to be parasitic lifeforms that have taken over a human host. They are ambush predators and seem to be able to lie in wait for years. I don't think they are true zombies, because they don't seem contagious, they don't try to bite, and they create growths in the host. While the growths alone don't disprove them being true zombies, it would match up better with parasite in my opinion. It doesn't seem contagious, as you can take a hit and not doom everyone, as well as the fact that someone survived a fight with it and was able to write a message, and the body is nowhere near there, if he did turn into a zombie. Considering the body was still there after 25 years, the other survivors are unlikely to have gone zombie hunting, although he could have made it back and gotten killed there after turning. You don't become infected, but this is almost definitely for gameplay reasons. I'm still using it for evidence, though. It is indeed infectious, as shown by a later record made by Seth. He made his way to the infirmary with strange symptoms and was quarantined. Zombies also seem to have the ability to regenerate, making them possibly impossible to kill. This is highly worrying, as the rescue team may end up dying or even spreading a world-ending plague to the rest of the world. The first message from Seth appears on the 21st of February, 1989, his next record appears on the same date. This means at most it took a little less than 24 hours for symptoms to appear, but likely only a few hours. The low dormancy period is both good and bad, as it quickly allows for victims to be recognized and quarantined, but likely means the pathogen or parasite replicates extremely fast and will likely be incredibly difficult to contain, especially after turning. Depending on the severity of injuries sustained by Murphy (or Thompson? WHO IS THOMPSON S.?!) the dormancy period could be much longer, maybe a few days. I have no idea whether it would be infectious during that time.
In Security breach, there appear to be several zombies who are fairly new, less than a day old potentially. The bodies drowned aren't zombies, but other then that placement seems random. Proximity to the "nest" appears to have some impact, although there was at least one corpse near the nest that didn't bear the infection. It again seems to be some sort of parasite that may bear some relation to the growths on the wall, and infection does not seem to be guaranteed from interaction or death with the other organisms, it would make sense if the zombies infect everyone they kill, but given that the one man managed to survive his encounter for at least a few hours, it may be a pathogen rather than a parasite, or a really small parasite, at least at initial infection.

It exhales a corrosive substance. Ambush predator. Very little observations. Possibly immortal, I don't really know, I didn't stick around to find out. It seems to be the same species as the scratchers, they are released from it when it is threatened. They have the same surprising ability to regenerate as the zombies do.

Ambush predator. It can move surprisingly fast. It is commonly found near dead bodies or supplies. I don't know if it is an intentional tactic or not, but it is effective. It blends in well with the cave wall. The dead bodies sometimes found near them are possibly past victims indigestible parts.

I should be able to name one of these things after someone, at least in my head-canon. Neil DeGrasse Tyson seems like a good option, considering these are supposed to be alien in origin (and I also named something else Formido Reptans). Anyway, these are ambush predators. They lie in wait, usually on the ceiling or wall, and then lunge and spit acid at anyone who gets too close. They are volatile, spraying acid in a large radius on death.

This seems to be the most bug like of the non-flying creatures, the "spiders" and macrocheirants appear to be more closely related to crustaceans. It doesn't actually appear to be an ambush predator, with no camouflage to speak of, especially considering it is sometimes aggressive in hunting down the player. Its acid sack is stored in the head and will explode violently if ruptured. Despite occasional aggression, they appear to be quite skittish most of the time, although they block the path due to their defensive use of acid, and therefore can't really be left alive by Murphy. They bare a large resemblance to spiders, with many legs and three body sections. No noted symbiotic or reproductive association with other organisms.

There are a couple of creatures that you can find that aren't in the game. I've found a couple, but only one in stuff released by the dev, so I'm only going to talk about that one. The Therianthrope seems to be a mutated mass of at least one human body and the growths on the walls, possibly stimulated by radiation from experiment rooms or the reactors. No observations on behavior.