Pluto was named after the dwarf planet Pluto
which was discovered in 1930, the same year that the character was introduced.,

Pluto Clipart      Animated Pluto

  First appearance:
The Chain Gang, 1930
Created by:
  Walt Disney,
  Norm Ferguson
Voiced by: 
  Pinto Colvig,
  Lee Millar, Sr.,
  Jimmy MacDonald,
  Bill Farmer
The pup first appeared in Walt Disney's short The Chain Gang, released August 18, 1930. However, it had no name. The next appearance was on  October 23 1930, in The Picnic. Here the dog is named, but it is Rover, not Pluto. It was in The Moose Hunt, released on May 8 1931, that the dog is called Pluto the Pup, the studio's original name.