I think, like Link and Zelda, Epona is recurrent - a different character upon the same archetype, with the same name.... possibly a descendant or reincarnation (whatever you happen to believe about that in the LoZ universe).
I've whistled and hummed "Epona's Song" at work. Sadly, it does...
*Ranch Worker Raises her Hand*
The life cycle of horses:
Horses grow up pretty quickly. Little filly Epona in OoT struck me as "not quite a year old yet, maybe a year" - a horse is an adult (young adult) at about two years. 1-2 years is when most people start training them. (Well, you do...