The key rests in what is said in the announcement of the coming of the plague in Exodus 9:3, “Behold the hand of the LORD will come with a very severe pestilence on your livestock WHICH ARE IN THE FIELD, on the horses, on the donkeys, on the camels, on the herds, and on the flocks” (NASB, emphasis mine). This modifies what is meant in Exodus 9:6 when it says that “all the livestock in Egypt died.” It was the livestock “in the field” of the Egyptians that died. Just as animals were taken into shelter in Exodus 9:19-20, some of the livestock, that was not “in the field” during the pestilence was spared. This is clear, not only from 9:19 when the plague of hail came, but also from 9:9-10 when the plague of boils came upon “man and beast.” The plague of 9:6 greatly diminished the Egyptian’s livestock, but then the boils and hail further destroyed the livestock of any “who paid no regard to the word of the LORD” but “left his servants and livestock in the field” (Exodus 9:21).
Kyle Pope, March 2011