"Then the Lord said to Moses, 'Go to Pharaoh and say to him, "This is what the Lord, the God of the Hebrews says: 'Let my people go, so that they may worship me.' If you refuse to let them go and continue to hold them back, the hand of the Lord will bring a terrible plague on your livestock in the field - on your horses, donkeys and camels and on your cattle, sheep and goats. But the Lord will make a distinction between the livestock of Israel and that of Egypt, so that no animal belonging to the Israelites will die."'" NIV translation
Pharaoh is warned again and reminded that the plague will not affect the people of God. The Egyptians used many types of animals in their pagan worship including a black bull named Apis and, once again, Pharaoh is being given the chance to surrender to the one true God.