I have to say, though, Dire Rats are somewhat redundant because there are dozens of real carnivorous, roughly mansized critters you could use to fill their role. Like, say, wolves. Or dingoes. Or rabid dogs. Or wolverines. Or lynxes. Or giant otters. Or some of the smaller bears.
Eh, they fill a niche that none of those other examples do... dark, dank, cave dwelling scavengers. Some of those animals might make their homes in a cave, but a few levels under the earth or beneath a ruined castle, and you wouldn't expect to find many wolves or rabid dogs. But rats? Yeah, you expect the rats among the filth.