Postillion: -A man who rides the left or lead horse of a pair, especially the lead pair drawing a vehicle, in order to better control the lead pair of horses. The postillion may aid a coachman with six-horse teams as shown in the illustration. In the case of a post-chaise he guides the team in place of a coachman by riding the leader.
From Travel in England
by Thomas Burke, London: B. T. Batsford, Ltd., p. 104: He [Prince Puckler-Muskau, who toured England in 1826] found our postillions smart and accomplished, and noted that they were all men of small stature and light weight (like a jockey). Postillions (or post-boys, as they were called, though many of them were grey-haired) had a livery of their own. This was a short single-color jacket, a shiny white hat, white cord breeches, top-boots, white stock [neckcloth], and yellow waistcoat with pearl buttons.
At all the posting-houses, horses in pairs were kept ready in harness day and night, and the post-boys themselves had to be fully dressed during the day if they were the 'next turn-out.' Most houses kept ten or a dozen post-boys, who went out in rotation.
Stanley Harris, in his Old Coaching Days
, quotes a set of printed rules that hung in the yard of a famous posting-house. One of them was 'That the first and second turn post-boy shall be always booted and spurred, with their horses ready harnessed, from eight o'clock in the morning until seven o'clock at night.' Georgian Index - Carriages