Mickey Mouse belonged to a guy at the stables where Soda used to work. He was an ornery creature, but Soda loved him and Mickey Mouse loved Soda back. Soda went to see him every day, and the normally difficult horse responded by being gentle with him and coming when he was called. As Ponyboy put it, "he may have belonged to another guy, but he was Soda's horse".
After awhile, the horse was sold, and Soda was broken-hearted, and "bawled all night long". Ponyboy summed it up well - "Soda never really wanted anything except a horse, and he'd lost his" (Chapter 3).