postillion

from WordNet (r) 3.0 (2006)

postillion
    n 1: someone who rides the near horse of a pair in order to
         guide the horses pulling a carriage (especially a carriage
         without a coachman) [syn: {postilion}, {postillion}]

from The Collaborative International Dictionary of English v.0.48

Postilion \Pos*til"ion\, n. [F. postillon, It. postiglione, fr.
   posta post. See {Post} a postman.]
   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}.]
   [1913 Webster]