Lenny the Beaver first appears in "Up a Tree." He has only a few lines in which he suggests using a trapdoor or "something like that, with gravity." Once Finn's presence is realized and the other animals surround him, Lenny is the one to knock him out with the acorn.
Lenny the Beaver looks like a regular beaver. His body is covered in brown fur with a brown tail. Lenny the Beaver also appears to have jowls, similar to Jake. Underneath his dark nose, he has a pair of buck teeth. His eyes also seem to be a light brown, instead of the usual white.