We need more Wolf-copypasta-with-a-different-name-and-a-slightly-altered-size-and-speed #53203, not creatures with unique properties and gimmicks.
In other words, as I see this, this is actually something that may very well be needed.
Because for as weird of a mythical world that Dwarf Fortress is, it sure doesn't contain a big
variety of
mythical creatures, who possess unusual and/or unnatural gimmicks and twists.
Like, we have.. Uh, what do we have again? I mean there's blizzard men, who are mortal and feel pain - but are otherwise almost invincible.
I can't really think of more, simply because it's not like there
are many.
Just as an example, you could just add another random critter, except it wouldn't be a completely dull critter. The critter would be, say the Wyverns somewhat inspired from HoMM3;
(Because it's personally my favorite take on "Wyvern"s.)
They would be rather strong, fast critters that fly with strong claws that cut deep when they decide to slash your Dorfs. (So far pretty average.)
But then they would have a poisonous bite that would cause strong, short-term dizziness and moderate pain, with the long-term effects being strong nausea and severe dizziness.
Nothing particularly interesting, it's just another critter, after all. But it's a critter that temporarily disables any dwarf it bites, and also it's a big, flying lizard.
How often do you see huge, flying lizards suddenly decide that they want a chunk of Urist McDoesStuff Quality Dwarf Meat?
The point here is that it doesn't have to be wildly mythological and way off the charts creatures. It can be, but it might as well be something like what I have just a couple of lines up.
Just something that has some properties. In this case a flying pest that's somewhat bigger than a dwarf, the
gimmick with it being that it disables dwarves per-bite if they do survive.
I can't really see what would be so
terrifyingly gamebreaking about adding big flying lizards that have a poisonous bite and who reside in swampy areas to the game.