>>715922787
Good question! Basically how much effort they're putting in. Draft horse pulling something everyday. An actively racing race horse (besides hay, thoroughbreds eat shitloads of grain between races, even mine that wasn't racing). A horse taken for trail riding every few days.
It's no different from you doing any exercise whether for work, to lose weight, or have gains. Pic related. Its a Quarterhorse (the most common breed of horse in the US and horse of choice for cowboys) but in competition for having muscles!