Mickey was originally created as a replacement for Oswald the Lucky Rabbit, an earlier star created by the Disney studio. However, when Oswald was sold to Universal Studios, Disney had to create a new character to star in their subsequent animated shorts. One day, during a train ride, Walt desperately wanted to come up with a money-making character to replace the one he lost, Oswald, whom he loved dearly. He had visions of a mouse in the back of his head (he had previously made silent cartoon shorts with animated mice). He wanted to name his new creation Mortimer Mouse, but his wife Lillian Marie Bounds thought the name was too pretentious, so she suggested he change it to Mickey Mouse which he did (Mortimer Mouse eventually became the name of one of Mickey's rivals). Mickey was also given a girlfriend named Minnie Mouse, who stared along with him in his cartoon shorts.
Mickey and Minnie debuted in the cartoon short Plane Crazy, first released on May 15 1928. The short was co-directed by Walt Disney and Iwerks. Iwerks was also the main animator for this short, and reportedly spent six weeks working on it. Hugh Harman and Rudolf Ising were credited for assisting him; these two had already signed their contracts with Charles Mintz, but he was still in the process of forming his new studio and so for the time being they were still employed by Disney. This short would be the last they animated under this somewhat awkward situation.
Plane Crazy apparently failed to impress audiences, and to add insult to injury, Walt could not find a distributor. Though understandably disappointed, Walt went on to produce a second Mickey short: The Gallopin' Gaucho. It would not be until Mickey's third, probably most famous, and first sound cartoon Steamboat Willie, that Mickey began to gain the popularity that he has today. The short's original release date - November 18, 1928 - was later made Mickey's official birthday.