From the Super Mario Wiki, the Mario encyclopedia
A Spooky is a creature wearing a ghost disguise only found in Super Mario World 2: Yoshi's Island and its remake Yoshi's Island: Super Mario Advance 3. They float near the ground, occasionally "jumping" to briefly gain more altitude, especially when they are nearing the player. If they reach a wall while floating up a slope, they will stop instead of turning around. Yoshi can stand on them and let them carry him. Spookies are capable of stealing Baby Mario, but to do this, they need to land directly on top of Yoshi.
If Yoshi attempts to eat them or attacks them with a Ground Pound, the disguise will disappear and the green Shy Guy under it is revealed. However, if Baby Mario has been stolen by the Spooky, it will be a Bandit instead. Oddly enough, a Spooky is one of the few enemies Yoshi cannot defeat by breathing fire.
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