The Collaborative International Dictionary of English v.0.48:

Outrider \Out"rid`er\, n.
   1. A summoner whose office is to cite men before the sheriff.
      [Obs.]
      [1913 Webster]

   2. One who rides out on horseback, beyond the main group, e.
      g., as a scout. --Chaucer.
      [1913 Webster +PJC]

   3. A servant on horseback attending a carriage.
      [1913 Webster]

WordNet (r) 3.0 (2006):

outrider
    n 1: an escort who rides ahead (as a member of the vanguard)