Bonooru

The Scarecrow (Or Bonooru) is a mysterious character that appears in both The Legend of Zelda: Ocarina of Time, and The Legend of Zelda: Majora's Mask. His appearances are more numerous in Ocarina of Time, appearing (along with a companion Scarecrow called Pierre) besides Lake Hylia in the childhood sections of the game.

If, as Young Link, the player shows Bonooru the Ocarina, he will ask for a tune to be played, to see if he can memorize it, this song becomes the Scarecrow's Song. When Link is an adult, playing the song to Bonooru again will result in him offering the player the services of his friend Pierre (the other Scarecrow who is now absent from Lake Hylia and wandering Hyrule)

Utilizing the Scarecrow's Song, the player can now summon Pierre in specific areas, Navi will usually give the player some indication of where these areas are by turning green and targeting an apparently blank area of the landscape. When the player plays the Scarecrow's Song, Pierre appears from the ground and provides a Hookshot or Longshot target for Link to use in order to reach previously inaccessible areas.

In the sequel, Majora's Mask, the Scarecrow appears in only a couple of places, seemingly confined to the Astral Observatory's lower floor, and in the Trading Post in West Clock Town. He can once again be taught a song to summon him in either of these places, but his uses in this sense seem limited.

However, the Scarecrow does teach Link two very important songs: The Inverted Song of Time and the Song of Double Time, which slow down or speed up the flow of time in the game respectively, allowing the player either more time to complete their quest, or less time should they wish to speed it up for any given reason.

Trivia
There is an unresponsive scarecrow in Twilight Princess, found in Faron Woods, who may be a reference to the one from OoT and MM.