Pluto first appeared in the 1930 Mickey Mouse cartoon The Chain Gang as a bloodhound on the trail of escaped prisoner Mickey Mouse. The bloodhound character was adapted into Minnie Mouse's dog, Rover. His name was later changed to Pluto and his owner to Mickey Mouse, making him Mickey's best pal.
Pluto is also considered one of the first Disney characters to break out of the "rubber hose and circle" formula style the studio had relied on; the dog's design gave him the appearance of actually being round instead of flat. In addition, Pluto is one of the first cartoon characters that is actually shown to have thought processes through the use of character animation. His thought processes are showcased in a landmark scene from 1934's Playful Pluto, in which Pluto becomes stuck to a piece of flypaper and attempts to figure out a way to get himself unstuck.
Audiences so took the loose-limbed hound to heart that, even as continued to co-star in many of Mickey's films, he was given his own series of solo-starring cartoons, starting with Pluto's Quinpuplets which came out in 1937, but even before that he became the only Disney standard character to star in his own Silly Symphony, Mother Pluto in 1936. Pluto has a veritable kennel-full of supporting players, including his nemesis, the bullying bulldog called Butch, and not one but two canine-cuties as sweethearts Fifi, the temperamental Pekinese and also Dinah the long-lashed dachshund. Interestingly, in Pluto's Quinpuplets, Pluto and Fifi are seen as "Mr. And Mrs. Pluto," the parents of five mischievous mutts.