Yeah, someone else stole a creation of Walt's ( i think it was
Oswald the Lucky Rabbit) which pushed him to make his own studio so he'd have control over his own intellectual property.
It's also worth remembering that if Disney doesn't enforce it's trademarks (which could include things like Mickey's likeness), they lose control of the mark, which is why they might sometimes go after people in a callous way. They have to ensure the mark isn't genericized. Look at what happened to Hoover, Aspirin and Thermos.