Rattus Faber
From The Fifth City Wiki
"You have rats: and some of the ratholes have been dynamited open, not gnawed. An infestation of L.B.s, aka rattus faber! Viciously intelligent rats who set traps for humans! No landlord or Constable will tangle with these."
Rattus faber are an unusual breed of rat: they can make tools - even complicated weaponry, like rifles and explosives - and generally operate like tiny humans. There is a large population of these on Pigmote Isle, though that is not their location of origin. Where exactly they came from - or how they evolved from normal rats - is unknown.
By the way, "L.B.," their colloquial nickname, is just short for Little Bastard.
