In March of 1990 Nintendo Power published a little four page behind-the-scenes article on the creation process behind Super Mario Bros. 3. It's an interesting little article that has largely been forgotten over the years, as I've never seen any of the little tidbits and factoids in the piece resurface in other places (such as the existence of a centaur power-up instead of the raccoon leaf).
"I'm sorry to say there isn't a funny story behind why we chose the Raccoon tail," Miyamoto remarked. "We thought the Raccoon tail worked best from a practical point of view and it fit right in with Mario's style. It created some great new game possibilities."
It's written for the magazine's young audience, but don't let that keep you away. It's a little light reading as you pack your suitcases for E3.