A Metal Boo is the result of a normal Boo coming in contact with a Metal Mushroom. They can still fly, but they fly very slowly. They can also destroy anything they fly into despite their speed. They are not white, they are gray, and they are indestructible. they have very evil metal eyes, and a special blue tongue of fury that can lick Mario and cause damage to him.
Some Metal Boo's will appear in Mushroom Kingdom War IV: The Revenge as enemies.