Horseshoe Grass

From Zelda Wiki.org

This is grass that grows around Hyrule that is in the shape of... you guessed it, horseshoes. As human Link, you can pick this up and blow on it to call Epona with her special song. As wolf Link, you just stand by it and howl the song and Epona will appear like normal. Later, after restoring Ilia's memory, she will give you Ilia's Charm , so you can call Epona whenver desired.