Block Boo

A Boo Buddy Block is a strange combination of a Boo and a Block only found in Super Mario World and Yoshi's Story. 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. They also appear in Yoshi's Story.

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