Edit: These things work exactly like hostile wild animals/thieves. The second they get out of their cage military will kill them and any cilvilian doing anything around them, including hauling them, runs like hell, so there's no real way to turn them into cattle, from what I can tell.
I've heard of a 40d fort that breed the unicorn mounts leftover after a broken elf siege, locked each one in an individual 1x1 cell, then released them when the mother had kids to be captured in individual cage traps, from which several could be butchered by the military and turned into roasts.
Dunno if that still works in this version.
---New Update---
I've taken Fleshforge's suggestion and created files that only contain one entity, so that the user can mix and match the entities he desires. The creature files are still undifferentiated, because the game won't generate the extra ones if the appropriate entity doesn't exist, so there's no need to.
I haven't made any other major changes because there don't seem to be any major bugs or outstanding requests.
I also rewrote the readme so that it includes an index of the attacking civs, including a table of the seasons in which enemy civs attack. I've included it in this post.
===Seasonal Schedule of Attacks:
Spring Summer Autumn Winter
Stranglers x x x
Frogmen x x x
Tigermen x x x x
Blendecs x x
Nagas x x
Furies x x x
Hellfire Imps x x x
Warwolves x x
Elephants x x
Nightwings x x
***Challenge Mode enemies come in all seasons***Edit: it'll only come up right if you quote it.