Pluto is an animated dog and is a Walt Disney character. He is Mickey Mouse's pet dog, but unlike most other Disney characters, he is not anthropomorphized, which means he doesn't have any uncharacteristic human traits. He first appeared in 1930, in a Disney short called "The Chain Gang," where he was a Blood Hound who was chasing after escaped prisoner, Mickey Mouse and he did not yet have a name. A few months later, in his next appearance, in a short called "The Picnic," he was given the name Rover and belonged to Minnie Mouse. He wasn't given the name Pluto (or Pluto the Pup) or given to Mickey until 1931, in a show called "The Moose Hunt"
Pluto became a main character in 1934, when he was given a key role in a cartoon called "Playful Pluto." Since then he has been considered a main Disney character. He is also said to be one of the first Disney characters to appear to actually be round rather than flat.