Grass Whistle

Grass Whistles, which come in two varieties, are plants that grow in many places in the overworld of Twilight Princess. With them, Link can whistle tunes to summon either a hawk or Epona. As Wolf Link, Link can howl the appropriate tune when he's near either type of Grass Whistle.

Hawk Grass
Link can blow into Hawk Grass in order to summon a hawk, which he can use to solve certain puzzles. The hawk can be aimed and released to retrieve faraway objects or attack enemies. After which, the hawk will fly away, but it can be called on again. If Link calls on the hawk as a Wolf, the Hawk will be able to point out secrets in the area, such as in the Sacred Grove where you fight the Skull Kid.

At the beginning of Link's quest, it is necessary to use this grass to get Uli's crib back from a monkey. Near Lake Hylia, when Lanayru Province is covered by Twilight, Wolf Link has to fight and tame a Twilit Kargarok called upon by Hawk Grass which he can use to call it again anytime. Link can ride the Twilit Kargarok to access upper Zora's River. After the Twilight is lifted, the Kargarok is required to play Plumm's minigame, which he can only do as a wolf.

Horse Grass
Horse Grass, which is shaped like a horseshoe, is used to call Epona. The tune Link plays on it is Epona's Song. After monsters invade Ordon, Epona will not respond to the whistle for a portion of the story. The Horse Grass becomes usable again for Link wrangles her in Kakariko Village. For the majority of Link's quest, Horse Grass is the only way of summoning Epona, but he can later receive the Horse Call to summon her whenever wherever she's allowed to roam.