I haven't been playing all that long, and most forgotten beasts have been decidedly un-"fun", until today where I killed a large scorpion dude covered in feathers, poisonous sting and all that. For months and months after that I've been having to deal with most of my pets coming down with some horrific wasting syndrome that essentially turns them in giant rotten blood balls, spewing miasma everywhere they go, but refusing to do die. Assuming that the scorpion stung them, I didnt really know what to do but kill or quarantine the sick pets so they didnt gross everyone out. After about a month passing without a noticeable case, most of my militia comes down with the same unfortunate syndrome.
It turns out a Towering theropod with lidless eyes and spare blue feathers and deadly blood had been around, but drowned itself in the underground lake I was using as a mud source for my deep fields. Around the same time I fought the scorpion I broke through the wall of the lake and watched a blood poured into the channel for a minute and thought nothing of it, I've noticed blood tends to show up from somewhere when excavating water sources without a good escape route my Urist Mcminerdorf.
A little bit of the forgotten beast/failed pool toy blood got on to the walls and stair well, and my fort has been tramping through it for a better part of three months, but it seems particularly aggressive towards livestock. They've just been walking down the stairs, brushing the walls with their hands like I do sometimes in a long deep stairwell...and wiping hemorrhagic goop all over themselves. I've lost about 15 dwarfs, most of them decent soldiers, to the stuff. And I didn't even fight the bastard.
On the plus side, my docs are learning how to not kill people quite so much!