Anything made out of soap. I had something made of soap before and when I downed it, it cleaned all my dwarves.
-.-
If you butchered it, and made soap from its tallow...
SOAPCEPTION.
Anyway, I once got a gigantic earthworm made of fire. It set the entire second cavern ablaze (no biggie - the only casualties were some ratmen that had wandered down there from the first cavern level after we wiped out most of their settlement with a 5X1 block of stone dropped onto their heads), came up to the first (which inexplicable had no moss, although the ground was muddied) and was promptly killed by an off-duty axedwarf recruit, who neatly bisected it by cutting its lower body off.
Yeah...that one was easy. I had to reroute the entrance into the caverns, though. Didn't want to risk my dorfs walking over the corpse and certainly didn't want them hauling it to the refuse stockpile (that's where all my bones and vermin remains are! I use those for making crafts! ...well, the bones, not the remains. The remains are sacrifices to Armok, thrown into the volcano to appease him while we wait to catch some goblins).
Then there was the gigantic humanoid made out of some kind of gem, which died in a fight with a war dog and its pup. It was fighting it on the edge of a pit that would have been a drop trap for gobbo invaders but accidentally pierced the caverns instead, and they both dodged into it. The FB died on impact, and the war dog struggled on for a few more moments with a broken upper and lower spine. So, casulalties: one dog (which was made into dinner).
Don't even get me started on the thing-I-can't-remember made out of water. A kitten took that one out. One badass kitten.