Mickey Mouse is a character from the Mickey Mouse series. He may be used in:

the Toy Box for Disney Infinity 3.0 only, and
all Toy Box Expansion Games (Disney Infinity 3.0).
He is a playable character in Disney Infinity 3.0, and was released at launch. He was revealed to be playable via leak. His persona from “Fantasia” was released for Disney Infinity 1.0 as “Sorcerer’s Apprentice Mickey”.