Boo

Boos are ghost enemies from the Mario series of games, debuting in Super Mario Bros. 3 for the Nintendo Entertainment System. They made cameo appearances in Link's Awakening where a pair of Boos, referred to as Boo Buddy, guards the Power Bracelet. They are killed by lighting the two torches and then swinging the sword at them, though the player could also just let them run away.

Boos are also trophies in Super Smash Bros. Melee and Super Smash Bros. Brawl. In Super Smash Bros. Brawl, on the stage Luigi's Mansion, if the mansion is destroyed entirely, a couple of Boos will float out of the ruins.

Trivia

 * In the Mario games, Boos can rarely be killed (only in SMB3 [only with hammers], Super Mario 64, Super Mario Sunshine, and Super Mario Galaxy), and hide when looked at. This is not the case for their Zelda appearance.
 * Because Boos appear only in Link's Awakening, which happens entirely within a dream, there is no proof for the actual existence of Boos within the Zelda universe, much like mermaids.