The Kittenrats were first discovered during Soha's ScreamFriends stream of F.E.A.R.. They were so named due to their abnormal size and being shaped much like a large kitten. During the initial encounter, the Kittenrat seemed to be harmless enough, but as numerous animated movies have taught us over the years, rats are never trustworthy. As such, many expect Kittenrats to play a larger, darker role in future world events as time goes by.
It has been suggested that Kittenrats hail from the same plane of existence as Business Centaurs, and may even have achieved sentience in that world. Here in our world, though, they're just big effin' rats.