Grass Whistle

Horse Grass, which is shaped like a horseshoe, is one of the two varieties of grass whistles (the other being Hawk Grass) in Twilight Princess. Link may pluck it and blow into it to call Epona. It only makes sense that the tune he plays with the grass is Epona's Song. As Wolf Link, Link can howl the tune when he's near the Horse Grass. For the majority of Link's quest, this is the only way of summoning Epona, but he can later receive the Horse Call to make her come whenever wherever she's allowed to roam.