i had a clawed pterrodactyl with deadly saliva, horns, and ribs outside its body. that one really worried me, i was making a fort underground and had not designed my defences to cope with an insane flying enemy like that. fortunately it went for my military dwarves who with a bit of a struggle managed to eventually take it down.
i got so much meat from butchering it, and have been trying to get my bonecarver to use its horns and teeth to decorate my weapons and armor, though it does not seem to work.
it gave me scales, but i havent been able to figure out if i got leather, although i had a tonne of qued leather items when it was butchered so for all i know half my fort is wearing it or storing sand in bags made of its skin. shame, i would have tried for a set of masterwork forgotten beast leather armor if i had been on my toes. and ill probably be making the 120 or so units of fat it gave me into soap, its seriously cluttering up my kitchen at the moment and dwarves are getting unhappy thoughts because all they have had to eat lately has been forgotten beast tallow biscuits. seriously, who complains about eating giant horned pterodactyl, it sounds delicious to me.
forgotten beast/titan corpses are an amazing windfall, their size means processing their remains takes ages though.