I believe the bull must be found in the wild to be able to ride it... I've not been able to locate a wild bull yet though.
well i think you have to go to a ranch, MacFarlane has one, and then lasso it after go ride it with the Y or triangle button
I've tried, they just get really pissed off at me... And I can't ride any of them, I've lassoed all of them to the point where they all tried to kill me at one point or another, especially the big hulking one. So perhaps it's not part of the game, and the screenshot that lead us to believe it would be part of the game was just Marston doing something stupid like me and getting paid back for it? :P
hmm this could be true.. I've still not been able to find a wild bull... Maybe it's like the train robbing thing.. R* had mentioned it was in the game, but when we got it.. it wasn't. Could also be a bug. -LB