Cavalry

Cavalry are soldiers riding horses. The earliest cavalry soldiers were in about 800 BC, when horses were finally bred big enough so that they could carry a full-grown man. During the time of ancient Greece and Rome, rich men were in the cavalry - they rode horses to battle. In ancient Greece, the way you knew a man was in the aristocracy was that he owned expensive war horses and could be in the cavalry.

But in the ancient world men did not usually fight from the backs of their horses. Usually they got off their horses and fought on foot. This is probably because cavalry actually isn't that useful against well-trained foot soldiers in the hoplite tradition. (Though Alexander the Great is certainly shown fighting on horseback).

In the Middle Ages, in both Europe and the Islamic Empire, more men began to fight from the backs of their horses, probably because foot soldiers weren't so well trained anymore. The cavalry continued to be an important part of any army until after the American Civil War.