骑马侍从;侍卫;先驱者
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.
outrider
n : an escort who rides ahead (as a member of the vanguard)