postilion
noun
pos·til·ion pō-ˈstil-yən 
pə-
variants or postillion
: one who rides as a guide on the near horse of one of the pairs attached to a coach or post chaise especially without a coachman
Word History
Etymology
Middle French postillon mail carrier using post-horses, from Italian postiglione, from posta post — more at post entry 3
First Known Use
circa 1640, in the meaning defined above