Boo Mushroom

The Boo Mushroom is a type of Mushroom which appears in Super Mario Galaxy. It's a white mushroom with a Boo-like face. When Mario obtains this Mushroom, he will transform into Boo Mario. It is the least used power-up in the game, found only in the Ghostly Galaxy, Sand Spiral Galaxy, and the Boo's Boneyard Galaxy.

The mushroom first appeared in the level titled "Luigi in the Haunted Mansion".

Trivia

 * Luigi is shown to dislike this item. When Mario uses it, Luigi mistakes him for a real Boo and is frightened by him.  Later, when Luigi uses it, he frowns.