From the Super Mario Wiki
Gusties are long, happy ghost-like creatures in Super Mario World 2: Yoshi's Island, Yoshi's Island: Super Mario Advance 3, Yoshi Touch and Go, Yoshi Topsy-Turvy and Yoshi's Island DS. They always travel in huge packs, allowing the Yoshis to eat them and get extra eggs. They can be used as stepping stones to reach higher areas. In addition to possibly knocking the baby off Yoshi's back, they can spawn endlessly in some areas.
They appear in levels where the terrain is bumpy and uneven. Gusties are prominently featured in the level What's Gusty Taste Like?
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