What is a job horse and what are his duties?

Horse races and to race and win

A horse job would be like a stable hand, or groomers. The stable hand would be in charge of taking care of the horse's necessaries, such as food, water that sort of things), and a groomer would be in charge of making the horse look fresh and clean. They'd brush the horse and bath them and may even touch up on the horse's "shoes".