The horses, donkeys, and camels do not refer to an ordinary travelling caravan but to the enemy's approaching army. The Persians included large numbers of donkeys and camels in their armies, both to carry the baggage and to confuse the enemy in battle. The watchman was instructed to listen closely when he saw this army approaching.
7 When he sees riders, horsemen in pairs, riders on donkeys, riders on camels, let him listen diligently, very diligently.”