From the Super Mario Wiki
Piranhabons are large Piranha Plants covered in Goop found only in Super Mario Sunshine. They resemble the head of Petey Piranha, and roll along the ground, leaving behind a trail of Goop as they do so. They can and will run over Mario without stopping if he is in their way. If Mario sprays them with F.L.U.D.D., he can clean the Goop off of them, though they will still leave Goop behind as they travel. When jumped on, they will splatter on the ground, leaving behind a puddle of Goop. Piranhabons can also fall out of the sky sometimes, exploding into Goop when hitting the ground.
Their name possibly comes from chocolate balls known as bonbons, which Piranhabons slightly resemble.