1. an escort who rides ahead (as a member of the vanguard)
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.
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.
WordNet (r) 3.0 (2006):
n 1: an escort who rides ahead (as a member of the vanguard)