The Horseman's Association are a group of individuals who originally provided horses for The Affiliation and were a group of elite ranchowners, stockliners and livestock owners who dedicated themselves to furthering their common interests.
The current President is Fran Lengyll.
Members of it changed their views wanting to join with John Farson and so killed off Pat Delgado, as he would never have consented to going over to Farson and secretly trained an small army of ranch hands, cowboys and town tough-boys to complete any work Farson wanted.
On the outside they remained loyal to the Affiliation receiving their orders from Farson discretly through Kimba Rimer.
Hambry Sheriff's Department sought out an allegiance with them to aid Farson.
In a sad sign of what Mid-World had become overall, the members of the Horseman's Association failed to see that, while resenting the upper-class status of those in Gilead, if they overthrew it, much of their own status would also be upended. Even under ideal circumstances, all contracts would be entirely renegotiated, and not neccessarily in their favor. That is, presuming Farson and his men simply didn't take everything outright. They may have thought they would retain the best of their current status plus new power; it seems likelier Farson would regard them as remnants of the same order he and his men were grinding to bloody pieces.