Boos, also known as Boo Diddlys and later Boo Buddies, are common ghost enemies in the Mario franchise that debuted in Super Mario Bros. 3. They are a type of white, spherical ghost with menacing and normally open mouths. They are known for their large tongues and small stubby arms. As you can see here, this little ghost is sad. You can cheer him up by adding this fanart Super Mario Boo Upset Sticker.