Epona is also the horse in the two Zelda games for Nintendo 64, Ocarina of Time and Majora's Mask.
To summon her in Ocarina of Time, you need to play Epona's Song on Link's ocarina. Once she arrives, her usefulness is limited to getting to places around Hyrule Castle quickly, practicing marksmanship, and shooting the ghosts that roam the area around the castle with Link's bow and arrow. Then you can capture the ghosts and sell them.
In Majora's Mask, you must go to Romani Ranch and the little girl will want you to pop the 10 balloons from Epona's back. The little girl will then teach you Epona's song, and you're ready to explore.
Majora's Mask information from Gamesmania: Majora's Mask Strategy Guide: http://www.gamesmania.com/html/zelda/show.asp?file=eponaback.