Aside from humans, there are many other forms of sentient life in the vast emptiness of space.
Treat them with caution, as not all may be as they seem.
Hostile
Xenomorphs - Aliens, also known as Xenomorphs to differentiate them from the general term, are extraterrestrial lifeforms that use other living beings as a host during their life cycle. With black carapaces and wicked sharp claws, powerful night vision and the ability to see people through walls, xenomorphs are extremely dangerous. They are intelligent and capable of speaking to one another via a hivemind. In comparison to Earth species, xenomorphs are closest to parasitic wasps that live in colonies with queens and require another lifeform to gestate inside.
Slimes - They feed off of the nervous systems of other sentient beings, absorbing their electricity for nourishment. Gooey.
Constructs - Shades and Constructs are slaved to their master's will. They must follow the orders of their master at any cost. They are capable of grasping intent, unlike synthetic beings. Shades as well as any constructs are EVA capable and can maneuver in space without outside aid. Should they die, they leave behind ectoplasma which can be used to make ghost-burgers. Constructs created by cultists automatically become cultists themselves, allowing them to identify their team mates, and even count for the escape objective. Clever crewmen who steal soulstones and shells can create their own constructs to fight the cult.
Statues - Made by the Wizard's spell Flesh to Stone. With the Staff of Animation their bloodlust is awakened and they start to hunt people, but can move only when they're not seen by anyone. They cannot be killed (or at least take A LOT of damage to die). Don't even blink about it.
Shadowlings - Shadowlings are light-sensitive alien creatures from space. They are mainly defensive, but can 'enthrall' humans, which is basically a euphemism for enslavement of their minds. They start off disguised as a normal crewman (they can still enthrall like this!) but they have to take time to 'hatch' to unlock their full potential.
Revenants - Revenants are ghosts that have been infused with strange magical (read: alien) energies and brought partially back into the mortal plane. They are invisible to the naked eye but can still be influenced by mortal means (i.e. lockers). In addition, simple machinery or animals will recognize a revenant's presence and be alerted by their existence. Revenants are typically immortal while invisible, but some of their spooky abilities will reveal them to the crew for brief amounts of time.
Neutral
Lizardpeople - Second class citizens, lizards are generally regulated to lower level work. Within a department a lizard staff member may enjoy anything from almost equal standing to obvious racism and disrespect, depending on their coworkers. They enjoy few advantages beyond their striking appearance.
Plasmamen - Another, infrequently seen, race of lower-class Space Station 13 employees, plasmamen can only survive by breathing plasma. All employees wear special spacesuits and helmets to protect themselves from the station's robust atmospheric environment.
Cyborgs - The latest, and greatest invention of Robotics experts at Nanotrasen, they are normally loyal to the AI. Who the AI is loyal to, however...
Monkeys - Sapajus robustus or the robust tufted capuchin. Nanotrasen usually ships a large number of these annoying cute creatures to the station, so they can be used in horrific medical experiments, virus testing, and other purposes. Research in matter compaction and rearrangement have resulted in the Monkey Cube, a molded, powdery cube that when exposed to water, results in a monkey. Sometimes known to use tools laying about.
Shades - The spirit of an unlucky crewmember, bound to serve the one who used the Soul Shard. Shades are fragile, but being recaptured into their soulstone heals them. All a shade can do is slowly drain life, remove clothes (and handcuffs) and spin chairs.
Golems - Though they function equivalently to their adamantine brothers the source of these traditional golems is implied to be magical in nature. Golems generally do not have masters when they appear, though they suffer all the same problems adamantine do otherwise.
Adamantine Golems - Lumbering people of shining stone, adamantine golems are born into the servitude of their creators. While some will immediately gift these golems with freedom others will keep them close as assistants, bodyguards, or attack dogs. Golems do not have a will of their own unless granted one by their creator, and are not directly responsible for their actions if commanded.
Slimepeople - Former humans who were converted to slime by mutation toxin. Many scientists working in xenobiology will undergo this mutation willingly to work closer with their slimes, but others may mass produce the toxin and try to push it on others. Their bodies are composed of a toxic slime, which shouldn't be consumed by other races.
Podpeople - Human blood, resurrected through a plant. Podpeople are rare to see, often only making an appearance if the cloner has been destroyed or the power is out for an extended period of time. They enjoy a significant benefit in healing but suffer huge drawbacks. Without a source of light, they quickly wither and die.
Flypeople - Walking abominations of mismatched DNA caused by teleporter malfunctions. Careful use and upgrading of the teleporter can assure those that dabble with it don't end up in this sorry state. Unlike other mutant races, there's absolutely nothing redeemable about existing like this, unless you just enjoy being a freak.
Skeletons - The skeleton within, unleashed! Somehow still alive, a skeleton is just a sack of stark white bones. Clattering spooks, due to their supernatural qualities they're almost always associated with wizards. Skeletons enjoy a wide selection of useful immunities, just don't expect the crew to ever enjoy looking at you ever again.
Shadowpeople - Cursed beings of darkness, shadows that cannot exist for long in the light. They skulk in darkened maintenance corridors or in workplaces with intentionally destroyed lighting. They are not intrinsically evil, but still invoke an understandable level of mistrust in pretty much everyone.
They share many of the passive effects of shadowlings, but none of their innate spells or abilities. They are far less a threat.
Zombies - Rotted remains of former humans, brought almost all the way back to life. Unlike the zombies you find in movies, these zombies don't crave your brains despite what they might be compelled to say sometimes. It's perfectly possible to coexist with these zombies, though understanding their slurring speech might drive some to wish it wasn't.
Jellypeople - Bizarre three eyed beings with innervated skeletons immersed in a jelly frame. Their appearance is ghastly and utterly alien, though for some strange reason they almost seem nostalgic. They are functionally equivalent to slimepeople in almost all regards.