Boo Block

Boo Blocks (also called Boo Buddy Blocks or Block Boos) are enemies that first appear in Super Mario World. They are Boos that have the ability to turn into blocks.

Super Mario World
Boo Blocks only appear in some of the Ghost Houses in this game. Similar to regular Boos, Boo Blocks will only approach Mario when his back is turned. However, when Mario is looking at them, the Boo Blocks will take the form of a solid block that Mario can actually stand on. This is required to clear a certain Ghost House.