We really haven't seen all that many Xenos in ER to date, but we've heard plenty through various sources. Note that some of this data may be completely inaccurate, and that we haven't actually seen most of these species to date.
The first 5 species were from this post. The rest are from various points when people tried to look up Xenos.
Vexile (Vex) Edit
An extremely common and prolific vermin that infests large starships and Tombworlds, as well as any planet foolish enough not to screen cargo. Vexile are 6 legged vaguely ovoid creatures with a heavy shell, small mass of tentacle arms and a great sucking maw of teeth. They seem to be capable of sustaining themsevles on almost any organic matter, regardless of origin, and are highly resistant to enviromental hazards such as poisons and radiation. They are pure scavengers, never attacking living prey, though stories of them swarming and devouring lone travelers are common nonetheless.
The origin of the Vexile is unknown-historically it was first discovered on a cargo ship near the end of the altered wars-though it is believed to come from a harsh and very inhospitable planet where it's hardiness and capacity for obtaining nutrients from almost anything would be prized. The Vexile's digestive system is uniquely efficent, producing only a very minute amount of waste, mostly consisting of unusable carbon compounds. Though non-sentient, Vexile can be trained and some planets make use of them as a sort of waste management system, feeding the garbage of the world into their endlessly hungry mouths.
It's questionable if these things can be considered lifeforms at all, or if they are more akin to naturally formed computers. Silicates exist only on a handful of very hot, rocky planets with very high gravity. The high gravity renders life almost impossible except on a nearly two dimensional plane; silicates are waffer thin and made of of milions of silicon based "molecular machines", complex molecules which can perform specific functions. These "machines" do everything from record data about their surroundings to replicate and repair other system. To what end they do this and if there is any greater motivation behind it then the barest drives of early life is unknown.
Bloat Trees Edit
Flora native to Hephestus, resembling large, black, limbless trees studded with large cyst like pods. These pods contain the seeds of the tree and take anywhere from 1-10 standard years to mature. The tree is self fertilizing and as such the species, for the most part, consists of natural clones. The pods can vary in size but their purpose is always the same: Bloat trees produce no fruit, rather they use explosive force to spread their seeds. Each of the cysts on the tree is filled with softball sized seeds and organic explosives; when the time is right the cysts burst and hurl their seeds up to 3 kilometers.
Naturally, bloat trees are considered a threat to any settlements and must be continuously purged from the surroundings of any town, lest the citizens come under sudden mortar like barrage from the very forests around them.
Blueroot is a plant orginally found on Earth-3. Mildly toxic and lacking any use as food or contstruction material it was left completely unused for several hundred years until it's unique medicnal properties were discovered. Now used to make "Blue smoke" cigarettes and cigars, exposure to which causes blunted affect and increased intuition. Addicts of the Blueroot, those with longterm and high level exposure, display a complete lack of emotion, survival instict and or drive yet have a level of insight that could almost be considered prophetic.
Wandering stones/Stone Squids Edit
Natives of the desert moon of Haldar. Quadrapedal beasts which wander the vast sands and wastelands of the moon on long,single jointed, club-like legs; they have no internal skeleton, rather they rely on extremely thick and tough exoskeletons, giving them the appearence of monoliths on legs. Their mouths rest at the very bottom of their bodies, beween their legs; to eat they lower themselves down, open their armored carapace and suck in whatever food they have found.
Wandering stones have no known natural lifespan; they are born at roughly the size of a pebble, but continue to grow as long as food is available. Some of the largest know specimens can tower over 100 feet tall with natural carapace several feet thick. As such, those who don't die while small tend to grow to the point of starvation over several hundred years.
A semi-sentient, tool using, cave dwelling predator. Has 4 arms, 2 legs, a crocodile mouth full of teeth, and is completely blind.
A spacefaring race of quadrupedal herbivores who manipulate complex tools via 3 prehensile tongues.
Appearance: 8 foot tall at the shoulder, 12 at the top of the head. Pigmentation varies, mostly dull greens with spots of brown and yellow. Legs are large and bulky, forward jointed and ending in a circular hoof. Body shape resembles, roughly, alpaca like proportions with a long neck and quadrupedal locomotion. Has no tail, 4 eyes equally spaced around the head and a solid lower and upper jaw with rows of teeth leading all the way back to the esophagus. Tongues, which are upwards of 4 to 5 feet or 1.5 meters, are kept mostly in the throat when not in use. Tends to be more ponderous than fast moving.
Diet: In nature it is believed that their diet of fruit and leaves from tall trees resulted in the long, prehensile tongues for which they are known. Outside of nature, they tend to eat any plant material, with specific diets being dependent on location, personal tastes and beliefs.
Space traversal details: Use of artificial materials to create spaceships. Models vary, but are based around a fairly standard model of strapping large rocket engines to vaguely tube shaped things. Resemble early human rockets in many ways, save for the controls. As of yet they have yet to expand beyond their planet and a few colonies on other planets within their solar system. Manned Interstellar space flight is beyond their means currently, though they have launched probes. All observations of them have so far been from a distance and it is unknown if they have any realization of our presence.
Tools and weapons: Perhaps due to their herbivore origins, weapons, war and other overtly hostile actions and tools are much more rare. Those weapons and tools that they do possess are almost universally based around either use of their tongues for precise movement or their feet for power. As such, things such as screwdrivers are designed for tongues, while shovels are for feet. Guns, a relatively new invention for them, are based around being held in the teeth and fired via the tongue.
The Haebi are a difficult race to describe, seeing as they are so unlike humanity that most attempts at parsing the data into categories fails somewhat. Their apperance is highly mutable in just about every way, to the point that it's difficult to discern what's a tool and what is a purposefully bred subspecies. Their only defining trait is their generally fungus like appearance and highly specialized natures. Soldiers, for instance, have no other purpose then to fight and every part of their body is molded toward that end with little to no extraneous bits. The same can be said of diggers, constructors, pilots, masons, and every other possible aspect of their society. Where humans would have a job, the Haebi have a subspecies.
When it comes to culture and leadership, we are also at a loss. Judging from the actions of the species as a whole, it's clear that they either have some sort of central intelligence, or , if they don't, that they were created or evolved in such a way that they would follow certain logical paths and actions via instinct. The flight paths of their spore worlds, for instance, are not random; they follow well reasoned paths, using gravity wells to speed up and slow down, as well as targeting more star rich areas of the galaxy, rather then drifting through vast empty spaces.
Attempts to demonstrate or disprove intelligence within any individual of the species have failed, but many believe this is simply because the individual has been taken outside their bounds of usefulness, and that standard tests of intelligence fail to illustrate their true capacities.
The origins of the Haebi, both their home planet or even their home system, as well as their purpose, if any, are unknown.
Hitchers are a much more simple species that are considered hostile merely by the nature of their method of travel and sustenance. Hitchers are one of the rare species that live in space without the assistance of external machines or tools, capable and in fact evolved so that they can live their whole lives in space. As such, nearly every aspect of their anatomy is vastly different from standard terrestrial life.
Hitchers are carbon based, but lack the heavy use of water within their bodies that is found in terrestrial life. They do not breathe in a standard sense, instead taking in any needed gaseous elements via breaking them down out of solids, much like a fish getting oxygen from water. However, hitchers do this all via ingestion, rather then respiration of any kind. As such, the spend almost their entire, multi-thousand year long lives feeding their glacially slow organic processes.
Their size varies greatly, mostly because of their ability to discard sections of their body in order to conserve energy and chemicals. As such, a Hitcher with an ample supply of food will grow to a great size, mostly via the addition of extra or longer limbs, greater movement organs or even an expanded brain. Those Hitchers which humanity has had contact with were those rare few who were so gorged upon minerals that they had the capacity to grow and use brains capable of communication.
From these few interviews, it's clear that their perception of time and conception of reality vary greatly from humanity's, with things such as the rise and fall of stars being something a single 3 generation family might witness. Oddly, these perceptions seem universal, even among those who have only recently grown their "first brain", as though such thoughts are instinctual.
Hitchers make their homes in asteroid fields, clinging to the stones and slowly devouring them over the course of hundreds of years. During this time they will store any minerals they can not metabolize in special movement organs located all over their body. Once their source of food as been expended, they will use these minerals like directional jets, shooting them out in order to maneuver to a new asteroid. The difficulty and hostile labels come into play when hitchers either accidentally or intentionally land on human ships and proceed to start eating them. Occasionally this can happen in swarms and pose a real danger to smaller ships, but a single hitcher is usually simply scrapped off and dumped back into it's environment like barnacles on sailing ships.
The stardrifters themselves are physically not that interesting. They're warm blooded tripeds with a strange sort of 3 way radial symmetry, at least externally and skeletally. They manipulate things with hands that have opposable digits, eat standard organic matter, blah blah blah. Whats interesting about them is what they do.
First off, the stardrifters have no home planet, at least none that we know of. Their entire race all live on individual starships that contain no more then 2 members at any one time. These starships are only a few dozen meters long, yet contain machines which can synthesize any needed food or compounds, treat illness or injury and produce any tool needed to aid their masters. Oddly enough, the stardrifter ships have no "Jump" capacity, no way of traveling faster then light, and as such they do all their traveling at sublight speeds. The Stardrifters reproduce completely asexually via the use of cloning technology within their ship. The machine takes a sample from the current "adult", scrambles it a bit, and produces a "Child" which is then raised by the adult.
The stardrifters, aboard their ships, wander the galaxy on preprogrammed flight paths, stopping on various planets while completely ignoring others. Once landed on a planet they proceed to build what they call a "Point", a large antennae like structure filled with an indecipherable arrangement of machines, sensors and transmitters, all of which seem to be functional but don't actually produce anything. And this is where it gets odd, because the stardrifters themselves have no clue what they do either. In fact, they have no idea what they're doing.
Interviews with the stardrifters reveal that they only know how to work the machines on their ships-because their "parents" taught them- and how to build the Points. Why they do this, what the purpose of the points are, how long this has been going on, how the machines and ship function, and pretty much everything else have all been completely lost to time through countless generations. And yet they continue, building machines they don't understand and drifting through the galaxy with no end in sight.
Crystalline Race (Extinct)Edit
Appearance: 4-6/ 2 meter tall, complex hopper crystals. Move via rapid deformation and reformation, believed to manipulate tools in similar manner, or may simply have grown tools from their own bodies.
Distinguishing traits: Little is known about their society except for their almost universal use of simple geometric solids. They appear to have mastered a sort of multipurpose metallic substance for use as everything from a weapon to a data storage medium. It is employed in almost every situation, save for their personal stasis pods, which employ more mundane substances. They also created a power source of unknown magnitude, origin and construction which allowed them to survive the destruction of their sun.
Cause of Extinction: The majority of the species died with the destruction of their sun, but the remaining survivors were killed by HMRC units when they removed a vital power source, causing the shutdown of life support systems.
Also natives of the Haldar, these creatures are most known for their "mane of arms", a phrase used to describe the six limbs which ring their upper torso. While their locomotion is actually quite ordinary and quadrapedal upon two rear legs and two of their front legs, they still make full use of the other limbs. Each limb is identical to the others, relatively thin yet stringy and powerful, with two "elbow" joints and one ball joint "shoulder". Each arm ends in a grasping, 4 digit "hand", the digits of which are all evenly arraged around the circular palm. The Sen-ni's many limbs allow it a great deal of freedom and dexterity, which it uses to easily traverse the endless cave systems which honeycomb Haldar and make up the great part of it's ecosystem.
The Sen-ni's main method of survival is to climb to a high point in the caves, hold itself up using the limbs coming from it's back, and then attract prey using the biolumenecent patchs on it's body. When prey wanders close it will drop and ambush it.
Will-o-the-wisps occupy a strange place in the food chain of Haldar. They are little more then 2 pairs of fleshy wings, a great bioluminecent bulb and a collection of razor sharp teeth and on their own are mostly harmless; as such they are easy prey for almost anything else. However, they also form great swarms of swirling, luminecence which can strip the flesh from anything they come across. They gained their name when several early explorers of Haldar lost their lives following a single individual of the species deeper into the caves, only to be swarmed later.