Released: 1995, Super Nintendo
Super Mario World 2: Yoshi's Island is a dramatically different game from Super Mario World. Rather than controlling Mario, the player controls Yoshi and has to guide baby Mario to safety. As with its antecedent, it has quite a bit of unused content.
Among the more notable content hidden in the game, there are sprites for baby Mario crawling, some scrapped transformations, and even a dragon coin.
There is even a little bit of art from when the game had a different art style.