The Scarecrow's Song is a song that the player makes by themselves. It can be used to summon a scarecrow that functions as a hookshot target.
To get the Scarecrow's Song, Link must first be a child and go to Lake Hylia. By the Fishing Pond there will be 2 scarecrows. Use the ocarina and the scarecrow will tell Link to play a song. Make one up and it becomes the Scarecrows Song.