You can also find info on the multiplayer horse racing available in Liars & Cheats.
RockStar news wire article here...
With Multi-player Horse Racing, the Liars and Cheats Pack brings one of the most fan-requested DLC updates to the world of Red Dead Redemption Multiplayer. Races support up to 8 players, and each multi-player contest is a cup-style tournament that consists of anywhere from 3 to 5 races, where players earn points based on their position of finish.