Horse manure. Look at the rest of the prophecy:
Isa 13:14 And it shall be as the chased roe, and as a sheep that no man taketh up: they shall every man turn to his own people, and flee every one into his own land.
Isa 13:15 Every one that is found shall be thrust through; and every one that is joined [unto them] shall fall by the sword.
Isa 13:16 Their children also shall be dashed to pieces before their eyes; their houses shall be spoiled, and their wives ravished.
Isa 13:17 Behold, I will stir up the Medes against them, which shall not regard silver; and [as for] gold, they shall not delight in it.
Isa 13:18 [Their] bows also shall dash the young men to pieces; and they shall have no pity on the fruit of the womb; their eye shall not spare children.
Isa 13:19 And Babylon, the glory of kingdoms, the beauty of the Chaldees' excellency, shall be as when God overthrew Sodom and Gomorrah.
The reason that men will be scarcer than pure gold in this passage is because the Old Testament God is going to stir up Babylon's neighbors against her, so that the Babylonians are put to the sword, down to the smallest child.
-ccm