Isaiah 21:7

The verse reads: And if he see a troop, horsemen in pairs (1 Kings 9:25), a troop of asses, a troop of camels, then let him hearken, hearken hard. This apparently is the expected sign that great events are on foot; when the riders are seen the watchman is to listen intently to discover who they are and what they are doing. The word for "troop" means always "chariot" (usually collective); here it must be used in the sense of "riding train" like the Arab. rakb. The procession represents the Persian army. "Asses" and "camels" are probably introduced as beasts of burden, although both animals are reported to have been used by the Persians in actual battle.