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From Unmariowiki

Boo, also known as BOO!, is a floating white ball lookalike thing which might be a ghost, but nobody's sure. Its name is derived from the frequently used interjection, "BOO!". While it's not sure if he says "boo" or "moo", Boo is also called Cow, but since Boo doesn't look like cow (only tastes like one), many prefer calling him Boo.

Sub species

• Peeka Boo
• Whoop-i-boo
• Winnie-the-Boo
• Mountain Boo
• iBoo
• Booing Gum
• Boo Can't Touch This, also known as MC Boo