Mutt is a dog who appeared on The Muppet Show as early as season three. He appeared with some other dogs and Loretta Lynn in episode 308 where they sang "Oh, Lonesome Me" as well as episode 517.
He also appeared in Country Music with the Muppets, providing a running gag in which he constantly has a question for Rowlf the Dog, which gets answered in the form of a musical clip.