There's Star Stable, but I'm not too sure if that's a full on role playing game or not.
While Star Stable is probably the best game to try and role play in. The game is not for role playing in the slightest. There's an extremely picky [not as bad as WolfQuest] filter, a small typing limit per post, and the fact the game is meant for questing and meeting people.
Though, often than most, you see "role plays" advertised on a daily basis, though if you're used to paragraph posting, or even just proper sentence structure, you may be disappointed...
Other than Star Stable, there really aren't other horse games that are circled around being a horse. Games like Furcadia [which is practically 2D and that game is really old fashioned], and possible IT servers with a horse model can be used. Otherwise, Feral Heart creativity is your best option.