Targum for Exodus 12:29

And it was in the dividing, of the night of the fifteenth, that the Word of the Lord slew all the firstborn in the land of Mizraim, from the firstborn son of Pharoh, who would have sat upon the throne of his kingdom, unto the firstborn sons of the kings who were captives in the dungeon as hostages under Pharoh's hand; and who, for having rejoiced at the servitude of Israel, were punished as (the Mizraee): and all the firstborn of the cattle that did the work of the Mizraee died also.