From what can be ascertained through various Alien captive recorded logs, abominations are captured human test subjects hybridized with alien DNA, intended to be used by the aliens as biological weapons. The transformation process turns the subjects into feral, menacing creatures - hostile to anything that is not one of them (including the aliens that created them) - and removes all discernible sexual characteristics, leaving them with an androgynous appearance.
Their known creation history and asexual appearance is reminiscent of super mutants, while their hostile and reckless behavior mirrors that of feral ghouls. All three species "evolved" from humans.
When an abomination spots a target, it will point at the target, wail loudly, and then move quickly into melee range. It attacks very similarly to a feral ghouls, swinging overhand when at range and clawing horizontally when close by. Abominations are a fairly standard opponent and are easily killed with alien weapons. When killed, their bodies often contain various conventional alien items. Sometimes, however, they contain items commonly found on the bodies of feral ghouls (a pack of cigarettes, pre-War money, harmonicas, human flesh, baseball gloves, teddy bears, bobby pins, etc.), which serves to reinforce the abominations' human origin.
The way that the abominations point and screech when they notice the player is a reference to the 1978 remake of the movie Invasion of the Body Snatchers. The alien "pod-people" in the film point and screech in the same manner, whenever they discover a real human.