Boo Buddy Block

From the Super Mario Wiki

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.

The Boo Buddy Blocks in Yoshi's Story were simply called Boo Blocks. They appeared solely in Stage 6-3: Ghost Castle, and instead of being a gray block they had pink and white stripes.

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

Boo Characters
Bigger Boo | Boolossus | Boos in Luigi's Mansion | Bootler | Franky | Herbert | Igor | King Boo | Lady Bow | Lahla | Peeka | Spooky Speedster | Stanley | The Creepy Steeple Boo
Boos
Atomic Boo | Balloon Boo | Big Boo | Big & Little Boo | Blindfold Boo | Bomb Boo | Boo | Boo Balloon | Boo Buddy (SMW) | Boo Buddy (YS) | Boo Buddy Block | Boo Buddy Snake | Boo Guy | Boo Man Bluff | Broozer | Calm Fishin' Boo | Circling Boo Buddies | Dark Boo | Dark Dark Boo | Disappearing Boo Buddies | Fishin' Boo | Li'l Boo | Mad Big Boo | Mad Boo | Red Boo | Sleepy Boo | Stretch | Terekuribo