What's smaller than a beaver and bigger than a rat?

For the past month, Skylar, Mark, and have been keeping a vigil for what we thought was a very cute backyard beaver swimming in our frog pond. Skylar went so far as to name him"Beavis" and the island in the pond was dubbed, "Beaver Island."  On beaver-patrol yesterday, however, I noticed that Beavis was swimming with the aid of  a long, thick, whip-like tail--no "ping pong paddle" here.  Turns out, Beavis is no beaver. No. Instead, we have a resident muskrat in our pond. He is much cuter than a rat--and about fifteen times bigger too.