Disney had just lost the rights to Oswald the Lucky Rabbit, his last cartoon character, to Charles Mintz. He and Ub Iwerks, his best animator, were trying to create a new cartoon character. Ub sketched a funny little mouse resembling Oswald, and showed it to Walt. Walt liked it and named it Mortimer. When he showed Mortimer to his wife she suggested to call it Mickey. So Walt called his mouse Mickey although he did call many characters who appeared in Mickey cartoons and comic strips Mortimer.
Oswald the Lucky Rabbit