Adam Clarke on Isaiah 21:7

And he saw a chariot, etc. "And he saw a chariot with two riders; a rider on an ass, a rider on a camel" - This passage is extremely obscure from the ambiguity of the term רכב recheb, which is used three times, and which signifies a chariot, or any other vehicle, or the rider in it; or a rider on a horse, or any other animal; or a company of chariots, or riders. The prophet may possibly mean a cavalry in two parts, with two sorts of riders; riders on asses or mules, and riders on camels; or led on by two riders, one on an ass, and one on a camel. However, so far it is pretty clear, that Darius and Cyrus, the Medes and the Persians, are intended to be distinguished by the two riders on the two sorts of cattle. It appears from Herodotus, 1:80, that the baggage of Cyrus' army was carried on camels. In his engagement with Croesus, he took off the baggage from the camels, and mounted his horsemen upon them; the enemy's horses, offended with the smell of the camels, turned back and fled. - L.