The Boss Key has been in almost every Zelda game after Ocarina of Time for the Nintendo 64 . Hidden somewhere in each main dungeon (except the first three in Ocarina), the player must find it in order to open the Boss Door which allows the player to face the dungeon boss. It's appearance changes from game to game.
In the two Zelda DS games the key was too big and heavy to fit in Link's pockets, so he had to carry it over his head instead. This required the player to make a clear path for Link so he could carry it to the door.
In Spirit Tracks there is a electric verision of the key that only a Phantom possessed by Zelda could carry. This game also had Floor Masters that would try to steal the key from you, if they got it, they would put it back to it's original location.