This is Flare Blitz, main character of the Pokemon crossover fic A Pony Out of Place.
 
Author’s description: Originally a Ponyta from the Pokemon world, he was dragged into the space between worlds during a fight between Dialga and Palkia. He was thrust into Equestria and given a pony form by and unknown entity. He met Twilight Sparkle and she spent weeks trying to find a way to help him home. After spending time in Equestria and getting to know its inhabitants, he made the decision that he didn’t want to return home. He had also begun to develop intimate feeling for Twilight, which caused a few issues of their own and that was when Arceus appeared to bring him back to his original world. It was agreed upon that is Flare could win an upcoming race, he would get to stay. He won by evolving into a Rapidash during the last stretch, but Arceus was dissatisfied, challenging him to a battle.
 
The match was understandably one-sided, but by utilizing a unique way of using magic, Flare was able to impress Arceus enough to allow him to stay. But due to the fact that he had evolved into a Rapidash, when the spell to return him to his pony form was cast, he became a Unicorn instead of an Earth Pony.