騎馬侍從;侍衛;先驅者
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)