The skill of managing horses, camels, and other common riding animals. Bizarre mounts such as ostriches, bipedal dinosaurs, and the like can be ridden at one Ability rank lower then normal.
Poor - Slid off the saddle from time to time
Average - What everyone can do - ride a paltry mare and not fall of in a low jump
Good - You can handle a spirited gelding and canter and gallop quite well, perhaps jump a short gap
Great - you compete in the local steeple-chases with a good change of making all the jumps
Exceptional - You are a world class horsman, able to jump most any normal obstacle, and handle almost any normal beast
Extraordinary - You are a jockey the equal of Fred Archer himself; you've probably won your share of national races and maybe palced (or won) in the Derby