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. They 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.
The Boo Buddy Blocks in Yoshi's Story were simply called Boo Blocks (since they were actually normal-sized Boos rather than the smaller Boo Buddies). They appeared solely in Stage 6-3: Ghost Castle, and instead of being a gray block they had pink and white stripes.
Boo Blocks also appear in Mario & Luigi: Bowser's Inside Story in the Energy Hold.
In Super Mario Sunshine, there are Red Boos which turn into solid platforms when squirted. These, too, are needed to complete certain levels.
