Rat from the movie
Rats are small, omnivorous rodents that resemble mice, but are considerably larger and more aggressive. They are occasionally hostile towards rabbits.
In the film, the Sandleford deserters take shelter in an abandoned barn, where they are attacked by a group of black rats. They manage to escape, with some inadvertent assistance from an owl which swoops at both groups.
In the TV series, rats play a major role in the final siege against Efrafa. During the climax of the third season's premiere "The Last Battle", they are among the many animals recruited by the Watership Down Owsla; they chew at the roots of the tree holding the Efrafan warren in place, causing it to collapse and cave in.