http://bibleapps.com/commentaries/isaiah/21-7.htm

21:1-10 Babylon was a flat country, abundantly watered. The destruction of Babylon, so often prophesied of by Isaiah, was typical of the destruction of the great foe of the New Testament church, foretold in the Revelation. To the poor oppressed captives it would be welcome news; to the proud oppressors it would be grievous. Let this check vain mirth and sensual pleasures, that we know not in what heaviness the mirth may end. Here is the alarm given to Babylon, when forced by Cyrus. An ass and a camel seem to be the symbols of the Medes and Persians. Babylon's idols shall be so far from protecting her, that they shall be broken down. True believers are the corn of God's floor; hypocrites are but as chaff and straw, with which the wheat is now mixed, but from which it shall be separated. The corn of God's floor must expect to be threshed by afflictions and persecutions. God's Israel of old was afflicted. Even then God owns it is his still. In all events concerning the church, past, present, and to come, we must look to God, who has power to do any thing for his church, and grace to do every thing that is for her good.
Barnes' Notes on the Bible
And he saw a chariot with a couple of horsemen - This passage is very obscure from the ambiguity of the word rekeb - 'chariot.' Gesenius contends that it should be rendered 'cavalry,' and that it refers to cavalry two abreast hastening to the destruction of the city. The word rekeb denotes properly a chariot or wagon Judges 5:28; a collection of wagons 2 Chronicles 1:14; 2 Chronicles 8:6; 2 Chronicles 9:25; and sometimes refers to the "horses or men" attached to a chariot. 'David houghed all the chariots' 2 Samuel 8:4; that is, all the "horses" belonging to them. 'David killed of the Syrians seven hundred chariots' 2 Samuel 10:18; that is, all "the men" belonging to seven hundred chariots. According to the present Masoretic pointing, the word rekeb does not mean, perhaps, anything else than a chariot strictly, but other forms of the word with the same letters denote "riders or cavalry." Thus, the word rakab denotes a horseman 2 Kings 9:17; a charioteer or driver of a chariot 1 Kings 22:34; Jeremiah 51:21. The verb rabab means "to ride," and is usually applied to riding on the backs of horses or camels; and the sense here is, that the watchman saw "a riding," or persons riding two abreast; that is, "cavalry," or men borne on horses, and camels, and asses, and hastening to attack the city.
With a couple of horsemen - The word 'couple' ( tsemed) means properly a "yoke or pair;" and it means here that the cavalry was seen "in pairs, that is," two abreast.

A chariot of asses - Or rather, as above, "a riding" on donkeys - an approach of men in this manner to battle. Asses were formerly used in war where horses could not be procured. Thus Strabo (xv. 2, 14) says of the inhabitants of Caramania, 'Many use donkeys for war in the want of horses.' And Herodotus (iv. 129) says expressly that Darius Hystaspes employed donkeys in a battle with the Scythians.

And a chariot of camels - A "riding" on camels. Camels also were used in war, perhaps usually to carry the baggage (see Diod. ii. 54; iii. 44; Livy, xxxvii. 40; Strabo, xvi. 3). They are used for all purposes of burden in the East, and particularly in Arabia.