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 Link could also just let them run away.

Trivia

 * Boos also appear as trophies in Super Smash Bros. Melee and Super Smash Bros. Brawl. In Super Smash Bros. Brawl, if the mansion in the Luigi's Mansion stage is destroyed entirely, a couple of Boos will float out of the ruins.
 * Boos cannot be killed in the Mario series (except in Super Mario Bros. 3, Super Mario 64, Super Mario Sunshine, and Super Mario Galaxy), and hide if Mario looks at them. However, this is not the case in their Zelda appearance.
 * Because Boos only appear 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.