From the Super Mario Wiki
Piranha Pests (Peropera Pakkun in Japan) are Piranha Plants species who appear in two games; Yoshi Story and Mario & Luigi: Partners in Time.
Piranha Pests simply resemble the bud of a Piranha Plant with a propeller instead of a stem or stalk. They have tongues almost as long as a Yoshi's, but instead of making eggs, they attempt to swallow the Baby Yoshi and shrink him temporarily. They can only be defeated with a Yoshi Egg.
Piranha Pests are an exotic variation of Piranha Plant indigenous to Stage 4-3, Piranha Grove. They appear to weave in and out of the many infamous Piranha Stalks in search of food.
They have a close resemblance to the Jumping Piranha Plant and Hootie the Blue Fish.
Mario & Luigi: Partners in Time
Mario and Luigi fighting a Piranha Pest
In this game, Petey Piranha can spit out Piranha Pests. These plants use their long tongues to swing on a Question Block, trying to slam into Mario or Luigi. If they're hit with a hammer a few times, they'll be defeated.