Adam Clarke on Proverbs 21:31

The horse is prepared against the day of battle - Horses were not used among the Jews before the time of Solomon. There was a Divine command against them, Deu 17:16; but Solomon transgressed it; see 1Kgs 10:29. But he here allows that a horse is a vain thing for safety; and that however strong and well appointed cavalry may be, still safety, escape, and victory, are of the Lord. Among the ancient Asiatics, the horse was used only for war; oxen labored in the plough and cart, the ass and the camel carried backloads; and mules and asses served for riding. We often give the credit of a victory to man, when they who consider the circumstances see that it came from God.