In Medieval England the tallest horses were ridden by people of the highest

In Medieval England the tallest horses were ridden by people of the highest rank who, from their lofty position, looked down on their humble subjects. From this we get the phrase to be on one’s high horse or to ride the high horse, describing somebody who behaves scornfully or appear arrogant.
    The sport of horse racing is said to date from ancient Egyptian times. Presumably even then horse owners would make bets as to whose horse was faster. The ancient Olympic games of Greece included what are thought to be the first public horse races, and harness-horse racing began some years later when horses were set to pull chariots. The Romans preferred the chariot race above all others.
    Organized horse racing dates from 12th-century Britain, when royalty encouraged this pastime, and horse racing is still called the sport of king. Betting is an integral part of the racing scene, and unfortunately many individuals find themselves backing the wrong horse. It means to make an error in judgment, and has been extended to various causes and candidates since about 1860.
    Occasionally one may, in placing a bet, confuse one horse with another. Probably from some such occurrence comes a horse of another color, meaning a matter quite different from the one at hand. For example, a man wants to buy your house and offers you a certain price. You feel the price is right and accept it. But then the man says, "I will pay a small part of it now in cash, and will pay the remainder over the next five years." Most likely, your answer will be, "Oh! That is a horse of another color," and you will break off the negotiations.