Not exactly an enemy, they were used as test subjects to be implanted with alien embryos containing a chestburster.
The basic default enemy. they are only armed with a pistol but can do long range damage.
The heavier version of the normal marine, they are tougher and armed either armed with pulse rifles or grenade launchers
recognizable by their hazmat suits, they are not as tough as the Spec-Ops and are armed with a short ranged flamethrower.
Does not directly harm the player, but disgorges a facehugger.
The aim for the facehugger is to latch onto the face and implant an alien embryo. They can be found within eggs.
These are baby versions of the nromal adult alien and are usually be found within patients or Purvis. If you are implanted by a facehugger and have not been able to find an autodock unit, then it is game over.
The basic adult alien. you will usually hear them hissing and breathing so that can help prevent them from getting the drop on the player.
Rarer than the normal adult alien, they can be distinguished by their blue exoskeletons. They are also faster and deadlier than the normal adults.
- General Perez