Esther 6:8 (ESV)

8 let royal robes be brought, which the king has worn, and the horse that the king has ridden, and on whose head a royal crown is set.

This man was to be given a royal robe and a royal horse. From these two suggestions, it seems that Haman had in mind a coronation ceremony through which he would be placed on the same footing as the king. When David let Solomon ride on his own mule, it was in order to identify Solomon as his successor (1 Kings 1:33). As to the royal crest mentioned here, ancient drawings depict the pointed, crown-shaped ornaments that were placed on the heads of the king’s horses.