Myth War is a MMORPG that features centaurs as one of the primary races that a player can use. It is basically a game devoted to creating a team and then attacking every other player's teams you encounter.
The game is free to play but, like Second Life, you will need to shell out some change to make the game more interesting.
I tried to find a decent video clip from the game showing the centaur characters. All I found was this disappointing fluff. (I tink I'm gonna thwo up now.) It sure keeps you from wanting to play the game.