Jamieson, Fausset and Brown on Isaiah 21:7

7. chariot, &c.--rather, "a body of riders," namely, some riding in pairs on horses (literally, "pairs of horsemen," that is, two abreast), others on asses, others on camels (compare Is 21:9; Is 22:6). "Chariot" is not appropriate to be joined, as English Version translates, with "asses"; the Hebrew means plainly in Is 21:7, as in Is 21:9, "a body of men riding." The Persians used asses and camels for war [Maurer]. Horsley translates, "One drawn in a car, with a pair of riders, drawn by an ass, drawn by a camel"; Cyrus is the man; the car drawn by a camel and ass yoked together and driven by two postilions, one on each, is the joint army of Medes and Persians under their respective leaders. He thinks the more ancient military cars were driven by men riding on the beasts that drew them; Is 21:9 favors this.