Xenomorph

The Xenomorph life-cycle begins with an egg, lain by a Queen. From this egg hatches the first stage, an aptly named "facehugger". The facehugger, which looks like a walking hand, has eight legs and a tail. A facehugger hunts down living beings, and attaches itself to the face or other bodily orifices of the prey. It then secretes a chemical to sedate its prey, and proceeds to implant an embryo into the new host. When impregnation is complete, the facehugger falls off and dies. Eventually the host recovers, and is able to resume normal operation. Over time, the host will experience hot flashes, body pains, and other flu-like symptoms as the parasitic alien inside it grows and develops. When the parasite matures, it begins convulsing, attempting to claw/puncture its way out of the host. The process usually kills the host.

After birth, the alien is small (depending on the size of host), and will grow to become an adult alien, with traits similar to its host. At this early stage of development the alien is called a "chestburster." The adult alien is a deadly, efficient drone, which works on acquiring other hosts for the facehuggers of its hive. It lives only to build the hive, and protect the queen. On occasion, a queen alien is born, which will grow to a larger form, and be capable of reproduction by laying eggs.
Xenomorphs are the deadliest fighters in hand-to-hand combat, being extremely strong and agile. Their speed is extraordinary, their sheer strength and instinctive killing ability unrivaled. They are, without doubt, the ultimate killer.

Xenomorphs are organized in a caste structure within the hive. There is a single queen alien, which lays the eggs for facehuggers. There are several castes of alien, including drone workers, warriors, and "royal guard" - the best fighters. The royal guard protects the queen and the center of the hive, where all the eggs are. The warriors defend the hive while drones collect hosts and bring them alive to the hive. The drones then imprison them in resin near eggs which are almost ready to hatch. Although strong, xenomorphs are generally fragile. A single well-placed pulse rifle shot can kill a xenomorph, although the difficulty in making such a shot is extreme. Xenomorphs work best when attacking in swarms, since they have no ranged combat and must get in close. However, in cramped close quarters with little or no ability for ranged combat, a single alien can tear through a group of humans.