To those of you who question your sanity, you more or less voluntarily signed away your humanity when you registered for DFs forums. So don't worry, you're perfectly stable here.
I had an idea for a critter, which sotra worked.
The Feral Humidis - Imagine an animalman that can't do the things animal men can in terms of being able to speak, learn, etc. Now, while slow to mature, at adult size they are 3 times faster than a dwarf, 6 times his size, and posses natural fighting abilities that would allow them to take on weapon masters and hold thier own with lady luck on thier side.
That was the basic one. Four limbs and head and whatnot. Doesn't seem special.
Now, make them seemingly eternally pissed off at everything, add an extra set of arms, have them breath fire, and a spiked tail attack.
Maybe for the dwarves, give them a siege weapon that fires large serrated disks a distance equivalent to a ballista, and a form of floating guts created by dwarves to essentially be living land mines, that explode into corrosive AKA flesh eating acid/syndrom, which exist JUST to be that. I guess kinda like tyranid spore mines.
And make hammers out of stone as an early defensive weapon.
And a corpse catapult, so you can make use of all those dead fish dissectors and cheese makers.
I believe these and previous ideas will increase DF's already hellish level of brutal to exponential levels, with invaders being crushed with flying corpses, sliced up by serrated disks launched from afar, and angry badgers meeting thier end to rock hammers, and a babysnatcher being found melted into goop when the alrt appears as th living mine blows up in his face. As an added difficulty, make said syndrom transferable to anything that walk through it barefoot/handle contaminated gear without gloves or whatever.
Congrats Urist, that living mine you trained just melted your hands cause you didn't wear gloves to pick up that goblin's fancy metal hat. Or Giant that waded through that odd green puddle barefoot to kill Urist as he ran with said metal hat, say by to your feet as they melt off.
I lost the raw for the humidis sadly, otherwise I'd be happy to share it.