From the Super Mario Wiki
Piranhabons are Piranha Plants covered in Goop found 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 path. If Mario sprays them with F.L.U.D.D., he can clean the Goop off of them. However, they will still leave behind Goop as they travel. When jumped on, they will splatter on the ground, leaving behind a puddle of Goop. It is best for the player to simply avoid them.
Their name possibly comes from the chocolate dessert known as "bonbons", which are chocolate balls, to which Piranhabons bear a slight resemblance.