From the Super Mario Wiki
Grim Leechers are enemies from Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3 that exclusively appear in the levels The Impossible? Maze and Mystery of the Castle? These enemies resemble purple ghosts with skulls on their faces. If Yoshi is a good distance from a Grim Leecher, they will wait until Yoshi comes near, and then hover towards Yoshi once he comes too close. If they come in contact with Yoshi, the Grim Leecher will kick Baby Mario off Yoshi's back and hop on. This will cause the game controls to reverse, and Yoshi will be colored pink.
A similar enemy known as Fuwa Rover appears in Kirby Triple Deluxe.
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