Zechariah foresaw a future time when all of the nations would worship the one true God. The praise of the Lord would be so widespread that even horses would have “HOLINESS UNTO THE LORD” inscribed on the bells on their harnesses. These are the same words embroidered on the hat worn by the high priest of Israel (Exod 28:36).
It was customary among the nations of Bible times to hang bells on the harnesses of their warhorses. This gave them a regal military appearance. The noise produced by the bells also may have conditioned the horses for the noise of battle.
Zechariah 14:20 – In that day shall there be upon the bells of the horses, HOLINESS UNTO THE LORD.