Block Boo

A Boo Buddy Block is a strange combination of a Boo and a Block only found in Super Mario World. When looked at, it seems to be a normal, gray block. But if Mario takes his eyes off it, it slowly begins to transform into a Boo! The are actually required to clear a certain Ghost House, as Mario must lure them towards him, turn around, and jump on and leap off them to reach a high ledge, where the exit door awaits.

In Super Mario Sunshine, there are Pink Boos which turn into solid platforms when squirted. These, too, are needed to complete certain levels.