From AliensRL Wiki
As expected from its namesake, the great many enemies in this game are simply called "aliens". Unfortunately for you, they are the only other organic lifeforms in the towers, so get used to handling them quickly and effectively.
The worker caste tends to be quite busy with its own affairs and focuses on whatever tasks to which it is currently assigned, though individuals will become aggressive if the player becomes close enough to become a direct distraction. In terms of combat ability, the worker caste is known for its high constitution and extremely pain bite attack: in many cases, it would be wise to ignore them as much as they ignore you.
Hunters are the predators of the alien race found in the towers, searching for any fresh meat and tearing it to pieces once acquired. They move very quickly and are equipped with a powerful tracing sense, but the hunter caste as a whole are ill-suited for actual combat situations due to their fragility. It is easy for hunters to approach your position, so stay alert and keep your strongest weapon at the ready.
Most aliens fall into the warrior caste: bred for combat efficiency, they form the armies of the alien race. While not especially fast or durable, warriors are numerous and, among the highest of their kind, deadly in all aspects. Approach cautiously and ensure that you are in a position that cannot be surrounded.
Certain aliens are considered far superior to the numerous hordes that one will normally see when engaging them. They are to be considered the highest priority when determining what to kill, lest they shred you in the blink of an eye.