Raticate

Rattata evolves into Raticate, which is just a bigger, fatter, shaggier, ratlier rat, complete with the scaly, wormy tail that makes rats look so cool in the first place.

Raticate, at one time, was also the only pokemon in the game that could learn "Super Fang" and "Hyper Fang," which in Japan were basically known as hatred fang and certain-kill fang. How great is it that these relatively mundane rat pokemon received TWO completely unique and special battle moves? Pretty nasty ones, at that. Just assuming not everyone here has read my old-ass "pokemon zoology" page, Raticate does have a more specific basis than just "a rat." It's quite obviously patterned more after Myocastor coypus, also called the Coypu or Nutria. This is a particularly large, amphibious rat bred in some countries for both its meat and its cheap fur, better known in Japan than it is in the states.

I'm not just assuming a connection here because of body shape or anything, either; Raticate's own pokedex says it can swim, and that only its hind feet are webbed. This isn't even reflected in Raticate's design at all, but it is a feature almost unique, among rats, to the Coypu.