Heading South on a White Bronco

Get a Hungarian-Half Breed from Mexico. You can find them wild near the "Cornas" of Diez Cornas of the map. Once you have one simply go to a hitching post and save it. After that head to Blackwater and kill enough people to get the law after you. Ride into the wilderness and kill the lawmen that follow you. Once you get rid of your wanted level it will unlock.