In which 1931 Mickey Mouse short was Pluto first called by his familiar name?

Pluto was first called by his familiar name in the 1931 Mickey Mouse short The Moose Hunt.

The dog had already appeared in earlier cartoons, including The Chain Gang, but those appearances did not yet establish the permanent name Pluto. In The Moose Hunt, Mickey takes the dog hunting, and the character is identified as Pluto. This short therefore marks an important step in the character’s development from an unnamed or differently named hound into Mickey’s best-known pet.

A common mix-up is to identify The Chain Gang as Pluto’s first appearance. That cartoon is significant because it introduced the dog who would become Pluto, but the name Pluto came later. The Moose Hunt also belongs to the early sound period, when Disney shorts were rapidly defining the recurring personalities and relationships of Mickey’s supporting cast.