On a train from New York to Californa, Walt was thinking of a replacement for Oswald Rabbit during the train ride & recalled a mouse he tamed at the Lauge-O-Gram Studio in Kansas City.
The memory inspired the creation of a mouse he named Mortimer.
Lillian liked the mouse but thought the name Mortimer sounded unrealistic.
She suggested a name that sounded more dashing: Mickey Mouse.
When Walt heard of the name, he loved it.