The mountain beaver is a medium-sized, muskrat-like rodent often lacking a visible tail. It is not related to other beavers or rodents but are called beavers because of their habit of gnawing down young trees. The mountain beaver has an extremely short, fur-covered tail, and otherwise differs from the muskrat by possessing five-toed feet. It is dark brown with a small white spot at the base of each ear.

In Oregon, are common in forested areas on the west slope of the Cascade Range west to the Pacific Ocean. They are, however, rarely seen as they spend most of their time in underground tunnels, emerging only at night to eat.