I recently rewatched Aliens, and I was thinking: could a Xenomorph be accurately modelled in Dwarf Fortress? This is actually a pretty difficult question, because Xenomorphs use other species for reproduction (for those who are unfamiliar: they lay eggs in the victim's chest cavity, which hatch and then burst out), and they take on aspects of the host species, so it may be that a Xenomorph which hatched from, say, a dwarf would be noticeably different from one that hatched from a kobold. The reproductive cycle could be recreated with creative uses of syndromes and reactions, for example if Urist McRedshirt gets a face full of alien wing wong, then a syndrom kicks in which means he has X hours before an alien bursts out of his chest.
Xenomorph hives would be randomly-spawning features in the cavern layers, most likely in the lower layers as appropriate to the level of Fun involved, with facehuggers in the outermost areas, some xenomorphs as you go further in and a Queen in the inner sanctum.
I can imagine Xenomorphs being extremely Fun, being fast, strong, tough creatures that have acid for blood, as well as the whole facehugger thing. A potential offset to that could be adding weapons used by the space marines in the films, although futuristic assault rifles and flamethrowers may be somewhat unbalanced against medieval weapons and armour.