Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. One who rides out on horseback. [Obs.] --Chaucer.

   3. A servant on horseback attending a carriage.

Source : WordNet®

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