How about D&D Aboleths? They are simply awesome.
I've got aboleths (they can civ like other underground civs, ie. bat men), as well as a great many other D&D creatures. Essentially I just went through the entire 3.5 edition array of monster manuals and just added ones I thought weren't really super weird (there are a crap-ton of really, really weird monsters in the later books).
I've finished work on the domesticated insect creature from the earlier post and they are really neat to play around with in Fortress Mode. The different castes all have wildly different abilities, bodies, and so on; I was essentially just trying to push the limit and discover what works and what doesn't when it comes to caste vs. general tokens.
What I found was really neat was that the last "life-cycle" caste of the creatures (which I called "slivers") I made can_learn and can_speak, which makes them behave (and your dwarves behave towards them) exactly like regular sentient creatures. They can gain skills, start with names, and so on; most hilariously they can arrive with immigrants with any actual skill; I had for example "Catten Somethingorother, Trueborn Sliver Surgeon". You can even designate them as nobles (can't designate rooms, etc for them, though).
Anyhow, keep the suggestions coming, I've already also whipped up some more creature variation templates (stone, glass, etc versions of regular creatures, so I can easily whip up, say, glass elephants or obsidian dragons without too much work).