The Xenomorphs are a species that is unmatched by any other, they have no eyes but can still 'see' picking up on electromagnetic pulse much like a shark. They possibly have lungs, but have no need to use them if they do. They can survive underwater for long periods of time pointing towards the fact that either they don't breathe or they have gills and lungs. Their blood is a highly potent acid. No one knows what type of acid their blood is but possibly it is sulfuric. Their exoskeleton is acid proof as are their internal organs meaning that not only are they immune to their blood but possibly every other type of acid. They seem to be able to survive in habitats that could easily destroy any other life form. They are extremely fast and deadly using the shadows to flank enemies and also using sheer numbers to overwhelm them as well. No one knows where the Xenomorphs came from, but it is possible they are a biological weapon. At the same time there has been speculation on a Xenomorph planet (called Xenomorph Prime). If the Xenomorphs were engineered then possibly it was by the Space Jockeys to win the war between themselves and the Yautja, or maybe the Yautja created them to simply hunt and establish what class the Yautja were to be set at. Let's here what you guys think of this.
*EDITED BY DEATHDROP*