Postilion
A postilion (or postillion, occasionally Anglicised to "post-boy") rider was the driver of a horse-drawn coach or post chaise, mounted on one of the drawing horses. By contrast, a coachman would be mounted on the vehicle along with the passengers.
Postilion riders normally rode the left (or "near") horse of a pair because horses usually were trained only to be mounted from the left. With a double team, either there would be two postilions, one for each pair, or one postilion would ride on the left rear horse in order to control all four horses.