The Alien, or Xenomorph XX121, is an endoparasitoid extraterrestrial species in the Alien Universe.
They are not fully sapient, but are predatory creatures with goals of propagation and defence of their species, and have observational learning and problem solving skills. They infect human hosts and an embryo grows in the chest cavity until it bursts out, violently ripping the chest open and sending blood and fragments of bone everywhere, killing theirs host and becoming adult in a matter of hours. They work with a hive-like caste system with drones, warriors and a queen.
They have the ability of crawling along ceilings and walls. They have great physical strength, to the point of breaking through steel doors. They have blade-tipped tails that they use as slashing weapons. Their blood is an extremely potent acid and is capable of corroding on contact almost any substance with alarming speed. Some parts of their bodies, like the fronts of their head, are armored, but they are vulnerable to some human weapons.