The Xenomorph XX121, better known just as Alien, are an extraterrestrial, endoparasitoid species with multiple life cycles, possibly originating from the planet Proteus (also known as Xenomorph Prime). One of the most deadly of all known alien species, these creatures need a host organism in order to reproduce. The appearance of the Xenomorph varies depending on its host. The human phenotype is generally around 7–8 feet, and roughly 136.0 to 181.4 kilograms with a long, muscular tail and large, curved, oblong head. The Queen of this species is generally twice as large (they can grow even larger, up to 100 feet tall, and stronger if given time) and possesses superior speed, strength and intelligence.
Little is known about their biology, but the typical Xenomorph starts life as an egg-like creature which, when detecting the presence of a potential host, opens up to release the second stage: a small quasi-arachnoid life form which launches itself onto the host's face and impregnates it with an endoparasitoid larva, which later consumes the host from within and exits the body as a vaguely worm-like organism, which then grows very quickly, developing into the adult Xenomorph.
Xenomorphs appear to possess a roughly similar intelligence to that of a typical primate, but have shown to learn relatively fast and utilize the situation around their environment. Once fully matured, they have great physical strength and agility. They are masters of stealth; a favored method of acquiring prey is to wait in a dormant state until an appropriate victim strays near, and then drop down silently from behind or use its tail to impale whoever walks by. The prey is generally blind to the fact that a Xenomorph is present, due to its propensity to camouflage itself within its nest walls or the surrounding artificial environment due to its bio-mechanical-like appearance. A Xenomorph will also use its pitch black carapace to its advantage, lurking in the shadows whilst waiting for prey.
Xenomorphs, in all stages of their life cycles, are known to be invulnerable and vulnerable to small arms fire, ranging from 9mm to 5.56mm. Firearms with the explosive tip rounds, such as the M41A pulse rifle, have been known to cause the creatures to detonate. High-energy beam weapons, such as the plasma weapons used by Yautja species, appear to be the best way to kill the Xenomorphs. With that said, they are not invulnerable to melee weapons, although such equipment is likely to be destroyed upon contact with their acidic blood, with a few notable exceptions.