It's not really a beaver. The mountain beaver, a.k.a. Aplodontia rufa, is a rather large rodent native to North America's Pacific Northwest region, digging its burrows in forests from southern British Columbia to northern California. Even when there are plenty of mountain beavers around, people rarely see them, since they're unobtrusive, nocturnal animals.
One of the most primitive of living rodent species, the mountain beaver resembles an oversized gopher more than a beaver, lacking the latter animal's prominent tail or aquatic habits. Its common name probably arose from trappers trying to sell pelts. The fur of the adult is dark brown, with a small white spot under the ear, while the juvenile mountain beaver has a lighter overall hue. It's quite a pudgy, thick-bodied little beast, growing to about 25-45 centimeters long with very little tail.
Like many human residents of the region, the mountain beaver is exclusively vegetarian, eating a wide variety of local plants. But its digestive system is inefficient in getting all the nutrients out of its diet, so the pseudo-beaver has to eat its own feces and run the stuff through for another cycle or two. Just try getting a hippie to do that.
But I digress. Mountain beavers are territorial animals, digging extensive burrows and defending them fiercely from rivals. These burrows are often identifiable by "haystacks" of gathered vegetation the rodents leave at the entrances of their homes to satisfy late-night hunger cravings. They like to gnaw on tree roots and devour crops, rendering them pests to their hominid neighbors. The primitive physiology that makes their digestion so inefficient also requires large amounts of water daily; this necessarily restricts their range to moist, rainy areas.
Aplodontia rufa is the sole species (and genus as well) in family Aplodontidae of order Rodentia. In other words, it's not closely related to any other living rodent. The species has existed, virtually unchanged, for 60 million years or so.
The mountain beaver bears the ignominious distinction of being the exclusive host to the world's largest known flea, Hystrichopsylla schefferi.