Before Disney cartoons used his familiar name, Pluto was called Rover.
The dog later known as Pluto first appeared in the 1930 Mickey Mouse short The Chain Gang. In that film he is an unnamed bloodhound helping guard prisoners. In the 1931 short The Picnic, the character is identified as Rover, making that an earlier name than Pluto.
The name Pluto became established in The Moose Hunt, released later in 1931. The change is easy to miss because the character’s design and role developed across several early shorts rather than arriving with a complete biography.
Pluto differs from many Disney animal companions because he is generally portrayed as a dog without human speech. His silent reactions, expressive face, and comic behavior became central to his identity. He is usually associated with Mickey, although stories also pair him with other Disney characters.