Heading South On A White Bronco

The Achievement
 is a secret Achievement/Trophy in Red Dead Redemption that is earned by evading US Marshals while riding a Hungarian Half-bred horse in single player mode.

US Marshals seem to be called out once your bounty reaches $2500, however lower bounties have been confirmed if the player's fame status cuts bounties in half.

Tips and Tricks

 * You will need to have broken a Hungarian-Half Breed found in Mexico. You can find them wild near the "Coronas" of Diez Coronas or south east of Escalera (marked on the in-game map by three galloping horses). Once you have broken one you can either take it to a hitching post to save it or purchase one from a general store.
 * Now that you have your White Bronco, accumulate a bounty of approximately $2500 (or $1250 if your fame is maxed out) or higher and evade local law.
 * The easiest way to do this is to simply enter a town and start slaughtering the townsfolk. Shoot one person to get the attention of the local law enforcement, and then proceed to take them all out. Eventually the town will run out of officers and your wanted level will disappear, leaving only your bounty. Eventually you will be alerted to a group of US Marshals that have picked up your trail.
 * Immediately start running away from them, using horse pills, apples, and health restoring items as necessary. Once you have evaded the Marshals the trophy/achievement will unlock.

Notes:
 * The title of the achievement is misleading as you do not actually need to make it to Mexico to achieve it.
 * Killing all of the Marshals will not unlock the achievement. You must outrun them.
 * Marshals may come after you with less than $2500 on your head, especially if you have maxed out Fame.
 * In Armadillo, it has been reported that Marshals will pursue you after a killing spree with a Bounty as low as $575 with maxed out Fame

Trivia

 * The name of this achievement is a reference to a police pursuit of OJ Simpson, after being charged with the murder of Nicole Brown and Ronald Goldman. Al Cowlings drove a white Ford Bronco SUV, with Simpson lying down in the back seat, and attempted to evade the following LAPD Officers. Notably, Simpson drove his SUV northbound on Interstate 405, not southbound.