List of Super Mario Kart pre-release and unused content
From the Super Mario Wiki
This is a list of pre-release and unused content for the game Super Mario Kart.
The game was initially developed as an original IP and starred a "guy in overalls". The game was retooled as a Mario spinoff after the development team replaced the player character's sprite with Mario and decided that they liked the look of the game better that way.
A user on the Digitpress forum bought a prototype cartridge and showcased its differences in a thread.