STAR STABLE!! STAR STABLE!! STAR STABLE!! STAR STABLE!!
But yea, Star stable is a really great game. Not a good one for much role-playing, but there are a few people that love to. There's also another game coming out soon called Riding high, it looks really exciting!
Star Stable is probably your best option... Though like Owlus said, it's based on questing and competitions instead of roleplay, for the most part. Even if you did find roleplayers over there I doubt it would work out, seeing as how most of the players there are younger, and the chat filter doesn't help much either.
There's a game I play called Howrse, though. It's a breeding-style browser thing, but they have forums where people host roleplays of various types. I can't tell you what those are like, considering I've never actually joined one, but my friend has and she says they're fine.
I think it's worth the try... I find it a tad too competitive, but if you're only there to roleplay that wouldn't be an issue.