postillon (n.)

also postilion, 1580s, "a forerunner," a figurative use, from Middle French postillon (1530s), from Italian postiglione "forerunner, guide," especially for one carrying mail on horseback, from posta "mail" (see post (n.3)) + compound suffix from Latin -ilio. The sense of "one who rides the near horse of the leaders when four or more are used in a carriage or post-chaise" is from 1620s.