One of the main elements of the setting of The Outsiders is the difference between Greasers and Socs. The gangs are divided along socioeconomic lines. The Socs are the upper class, and the Greasers are the poor kids. Mickey Mouse is Soda’s horse. The significance of the story is that Ponyboy, a Greaser, felt close enough to Cherry, a Soc, to tell her about the horse.
Mickey Mouse was a dark-gold buckskin, sassy and ornery, not much more than a colt. He'd come when Soda called him. He wouldn't come for anyone else. That horse loved Soda. He'd stand there and chew on Soda's sleeve or collar. Gosh, but Sodapop was crazy about that horse. (Ch 2, p. 39)
The story ends with Ponyboy commenting that the horse was sold, and being glad the Cherry does not comment. Soda was devastated when he lost the horse.
The point of the story is that Soda never really owned the horse. Unlike Socs, the Greasers are poor. Like the selling of the horse, they are at the mercy of more affluent people. Ponyboy was telling the story to describe an aspect of Soda, but ended up telling Cherry a story about himself too.
To read more: http://www.enotes.com/outsiders/chapter-2-summary
Hinton, S. E. The Outsiders,. New York: Viking, 1967. Print.
Mickey Mouse, in my opinion was shared to discuss how Sodapop felt. Telling this story emphasized not only his emotions, but the Greasers emotions as well.