Quit worrying about adding a back story.The horse ran from its new owners to its previous owner whom im guessing treated it a lot better than the new ones.
He had a bond with that horse. He let the horse go but never forgot him.
3 years later he found out the horse had never forgotten him either and still had that bond with him to make him want to reunite.
It's love. It's if you love something set it free. If it loves you it will return.