From the Super Mario Wiki
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Poinks are small floating pig-like creatures found soley in Super Mario Sunshine. They almost always appear in groups. When Mario gets near them, one of the Poinks will attach itself to F.L.U.D.D.'s nozzle. If the player then squirts water, they will start to expand, and if the player releases the R button, the Poink will shoot off in the direction Mario was facing. They are helpful in one of the episodes against Petey Piranha in Bianco Hills. Jumping on a Poink will also cause it to explode.
Switching to any nozzle except the Squirt Nozzle prevents Poinks from attaching themselves to F.L.U.D.D..