The Beaver appears in "Susan Strong." He frightens Susan and steals three marshmallows from Finn. In the storyboards for the episode "Susan Strong," the Beaver wrestled with Finn. A different-looking beaver is seen in the earlier episode "Storytelling."
The Beaver looks similar to real world beavers: it has brown fur, a flat, dark-brown tail, and buck teeth. The main difference is that its body is one shape and rounded unlike a real world beaver.