From the Super Mario Wiki
Cloud Drops are ghosts that fly in a short line vertically or horizontally in Super Mario World 2: Yoshi's Island, Yoshi's Island: Super Mario Advance 3, Yoshi Touch and Go, Yoshi's Island DS, and Yoshi's New Island. They appear in packs and in areas where Yoshi can turn into a helicopter. They create air traffic and are difficult to maneuver around, but they can be hit on the head, causing them to fly down like a hit plane.
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