Goop Boos are one time enemies appearing only in Super Mario Sunshine. They attack by throwing up goop. Once defeated, they drop one coin.
Goop Boos only appear in Super Mario Sunshine in Sierra Beach During two missions. Mario had to beat them and find the hidden shine sprite the first time. In the second mission (King Boo Down Below), they caused trouble in the casino.
Weakness: Water and Yoshi Acid