Xenomorphs

Xenomorphs are a parasitic alien race that serve as the main villains of the Alien film franchise as well as being portrayed as at war with the Predators in the Alien Versus Predator franchises - they are also very popular villains in comic books, movies and cartoons outside the regular Alien canon and are regarded as one of the most fearsome and recognisable sci-fi monsters of all time.

Unlike many of the alien invaders that came before them the Xenomorphs did not seek conquest or human slaves, instead Xenomorphs were depicted as animal-like killers of the highest order - extremely intelligent and cunning but guided by a desire to hunt, kill and breed rather than anything more complex.

Unfortunately for any species that gets in their way the Xenomorphs can't reproduce without a very gruesome method of parasitic infection in which alien "face-huggers" attach themselves to a victim and inject them with a Xenomorph seed that grows inside their stomach, eventually bursting out of the victim as a "chest-burster" - this is, of course, fatal to the host - the "chest-burster" then proceeds to grow into a full-grown Xenomorph a short while after being "born".

Xenomorphs have several adaptations that make them even more terrifying - thick exoskeletons and amazing speed are only the start as their blood is highly acidic and they can use their tails as effective whips or stabbing impliments plus they have an extendable tongue with a set of teeth on it that can be fired out with enough force to pierce a human's skull.

Xenomorphs also absorb a small amount of DNA from their host bodies before being born and this can result in unique breeds of Xenomorph being formed - such as the "dog xenomorph" or the powerful Predator/Alien hybrid that served as the main villain of Alien Versus Predator: Requim.

A final note on Xenomorphs is that they are usually portrayed as having a matriarchal system ruled by a single Queen, the Queen Xenomorph is gigantic and powerful - seen as the most powerful of all Xenomorphs, though their may be more than one Queen as several have appeared in the movies and associated media - all varying slightly in size and appearance.