Answer:
Walt's animators had ditched him for higher pay at his former employer's, the same ones that stole his character of Oswald the Lucky Rabbit from him. Determined to make a character of his own that he owned all rights to, he had animator Ub Iwerks (his employee/partner) draw up some possible characters they could use, going down a list of all kinds of animals. Ub was inspired by an earlier drawing of Walt done by Hugh Harman, where he was sorrounded by mice (Walt had owned pet mice when he was little) and decided to make Walt a mouse character. Walt's original name for Mickey was Mortimer Mouse, which Walt's wife suggested they change. Much later on, animator Fred Moore gave Mickey the skin-colored face and "normal" eyes as we see him today. In addition to that, the character was further defined by the writers and artists behind his shorts.