During the Athenian Sicilian campaign the cavalry of the Syracuse became the dominant weapon on the battlefield, stifling the forty thousand strong Athenian hoplites from gaining the advantage in the field. The Athenians in return raised their own cavalry and sent it to Sicily. They weren't the knights of medieval times though, they were small ponies, approximately four and half feet tall, and they lacked stirrups. Hardly frightening, except the Greeks of that time were tiny. Hanson writes:

These tiny mounts, mostly stallions, were only partialled protected with light cloth padding over the face, thighs and chest, and harder to ride than geldings.

It was hard to train riders to control the stallions as well as handle a spear and blade in battle. (more)
Cam Riley: South Sea Republic. Freedom, liberty, equity and an Australian Republic.