A Muddy Buddy's concept art
Muddy Buddy is a ridable creature in Super Mario World 2: Yoshi's Island. Unlike Poochy, the other rideable creature in the game, it can be fully controlled. It can walk over spikes, allowing Yoshi to get past spike pits. Your Yoshi can get off it by Ground Pounding.
It only appears in W5-4: Sluggy The Unshaven's Fort in the original game, but also appears in World 2's secret level, Mystery of the Castle?, in Yoshi's Island: Super Mario Advance 3.