In the song Moses and Israel sang after crossing the sea, what had the LORD thrown into the sea?

Answer: The horse and its rider

"I will sing to the LORD, for He is highly exalted. The horse and rider He has thrown into the sea."

“Then Moses and the Israelites sang this song to the LORD: “I will sing to the LORD, for He is highly exalted. The horse and rider He has thrown into the sea.”— Exodus 15:1 (BSB)