POSTILION
postilion, postillion
(noun) someone who rides the near horse of a pair in order to guide the horses pulling a carriage (especially a carriage without a coachman)
Source: WordNet® 3.1
Etymology
Noun
postilion (plural postilions)
A rider mounted on the near (left) leading horse who guides the team pulling a carriage.
(obsolete) A post-boy, a messenger boy, a swift letter carrier.
Source: Wiktionary
Pos*til"ion, n. Etym: [F. postillon, It. postiglione, fr. posta post.
See Post a postman.]
Definition: One who rides and guides the first pair of horses of a coach or
post chaise; also, one who rides one of the horses when one pair only
is used. [Written also postillion.]
Source: Webster’s Unabridged Dictionary 1913 Edition