Aggressive animals can also double as small sort of hunters/alarm system. Can't be infected by werebeasts, after all.
For cave dragons in particular, takes a while to grow up, tho - over 26 years to reach the size of a regular polar bear. They also have two thirds of the speed of a dwarf, but that fire immunity is neat in said fort. Overall, cavern beasties aren't especially fast.
Aboveground, there are few significant options
Saltwater Crocodile: Largest "Large predator", grows up in 3 years and can be mass-produced quickly, but are slow (17 km/h gait, compared to dwarf's 30 km/h)
Polar bear: half the size of crocodile, but moving at 40 km/h. More unusual to encounter, however.
Lion: quarter the size than the crocodile, but twice as fast as the polar bear.
Cheetah: Slightly smaller than dwarf, but moves at 120 km/h.
Anything with a shell: None are predators, but idea is that they're harder to kill, buying you time. That said, they'll retract into shell and stay still, which can be quicker to coup than something that runs away at high speed (but on the bright side, doesn't pull a dwarf with "pasture animal" job).
Note that giant version of the cats don't boast the speed, going by wiki on cheetahs. However, even giant cheetah is bigger than a horse, with giant lion being a head taller than a jabberer.
Out of these, I'd favour polar bears the most - not egglayers, faster than dwarf but not too fast. That said, this is all speculation.