Facehugger
The lowly facehugger is the first enemy to encounter, and it is by far the most common. These guys love to dwell in dark corners and other out-of-sight areas, trying to surprise victims. The sharp limbs of the facehugger can easily tear through your armor, if you are not careful. They are the most hated, most annoying, and least understood of all xenofauna.

Alien Slave Drone (Worker)
While many of the alien species collected and studied by the Department of Xenotheric Husbandry exhibit a lifestyle that might be characterized as "undesirable" in human terms, the honors for most wretched existence must certainly be granted to the slave-class of alien, xenotherium subservilia. These pathetic creatures, bred for submissive behavior and unswerving loyalty, will attack and kill without mercy when so ordered by their superiors. In such instances they will wage war with an insect-like vigor, heedless of personal risk, fearing the wrath of their masters more than any possible harm an enemy may inflict. Internecivus Raptus

Alien Dog
This xenofauna breed is produced when the host is a canine. These aliens are fast, fierce and extremely difficult to detect before it is too late. They have larger than normal tails allowing them to strike their victims. Despite this, they are noticeably smaller than the average drone.

Parasitic Alien/Snake Alien
The parasitic alien is a newly thought species of alien. It is roughly thought to be the size of a forearm, and when inside a host it lays its eggs. Then a chestburster snake alien is formed skipping all other phases as the final result. These aliens are extremely fast and lethal.

Alien Warrior
The footman to the entire xenomorph species. Its sole purpose is to protect the hive, and the queen from any incoming attacks. Like many of its other inhabitants it has similar features except is by far the most lethal killing machine in the galaxy.Footsoldier of the invading army, this alien myrmidon relies on telepathy for coordination with its troops, but is no less dangerous when encountered in isolation. Like many inhabitants of the Portal Dimension, it appears to be a colonial creature, harboring several varieties of organic parasites which complement its destructive nature. Most dangerous are swarms of "thornets," heat-seeking autonomous "insects" which breed in their host's paracoelomic cavity and are only too happy to attack and consume the first thing they see upon release. The gleaming partial body armor at first appeared to be the product of an advanced weapons technology; but closer examination of the occasional moribund specimen has shown it to be a biologically extruded exoskeleton, synthesized from a diet rich in certain xenotic minerals. This suggests that long separation from the Portal Dimension, and a diet deficient in alien ores, might lead to the eventual weakening and demise of the alien troops. It is not, however, recommended that Earth rely on poor nutrition to repel the invaders.

Exomorphs
Biological weapons developed by the old Weyland-Yutani corporation in order to defend against the increase of predatory animals on off-world colonies. These

Cygnus Bioweapons Grunts
These guys are not exactly happy to meet you. They are sent by the Cygnus Corporation in order to retrieve some of the specimens still available. They will work together to take everyone down.