Whom did King Xerxes order to lead Mordecai through the city on the royal horse, proclaiming his honor?

Answer: Haman

Haman had proposed these honors thinking they were for himself; instead he had to robe Mordecai and parade him through the square.

““Hurry,” said the king to Haman, “and do just as you proposed. Take the robe and the horse to Mordecai the Jew, who is sitting at the king’s gate. Do not neglect anything that you have suggested.””— Esther 6:10 (BSB)