In order to provide a large array of enemies for the players to fight, the game takes some deviations from the established Xenomorph biology. The differences can be explained in their nature as experimental subjects.
Facehuggers, Chestbursters, and Infectoids
One interesting note about the aliens in the game is the effect of their Facehuggers. Their gestation period is fairly long, but unlike other Xenomorph broods, the victim is often reduced to a zombie-like state (known as Infectoids), and will move around and attack intruders. In addition, in some cases a chestburster will suddenly emerge from an infectoid, killing the host and leaving the smaller (but faster) chestburster alien as a new foe.
The Alien Warriors are typically based on the designs in Aliens, with a humanoid build and ridges along its cranium, and typically a black exoskeleton, although some brown warriors are seen. This breed is also known to spit acid. Its main attack is the stinger on the end of the tail.
These xenomorphs are similar to the creature in Alien³ and are slightly more dangerous than the average Warrior. Of note is that this species does not spit acid while the Warriors do - This is reversed from movie canon, where the Warriors were never observed doing this, while the xenomorph in Alien³ was.
Also known as Praetorians, a name popularized in the later Aliens vs. Predator PC games. They are almost as large and powerful as the Queen, and are typically encountered more frequently the closer the player gets to the Queen. Royal Guards are capable of spitting acid.
A very large Xenomorph with a green exoskeleton and a hard, tan carapace on its head, back, and forearms. It sometimes moves/attacks by rolling itself along the ground in a ball. It gets its name from the chrysalis that it is seen gestating in when it first appears.
This alien is very similar to one that appears in an Aliens arcade game made by Konami. Both Xenomorphs were the first bosses of their respective games.
Another large breed with a purple coloration and large, sharp claws on its fingers. It is known for its great speed, making it hard to defeat or even keep up with. It is fond of using that speed to make dashing attacks in which it uses its namesakes to cause massive damage and slice its victims in half. The birth of the Razor Claws was a result of a genetic malfunction in the cocoon process.
Appearing at the fourth level and the last level of the game, the Queen is quite similar to her appearance as well as behavior in the movie Aliens. She possesses long, spindly limbs and a thin body with an extremely large and crested head. The inner mouth displayed by her species is especially long in the game. Much like her role in the movie, she is very large and powerful and quite hard to defeat.