The list of units
The first mounted unit you can build, Horsemen move fast and will withdraw from combat if losing (unless fighting another fast unit).
A city must have horses in its Strategic Resource box to build a horseman.
The necessity for speed and mobility in times of war was recognized even in ancient times. The infantrymen of some ancient armies would ride horses to get from one engagement to another, dismounting to fight when they reached the battlefield. Because of their speed, horsemen could also be used as scouts to pinpoint enemy positions. In the second century BC, the Chinese invention of the stirrup allowed this concept of mobility to go one step further. Stirrups provided the stability and leverage necessary to allow soldiers to fight from horseback, leading to the earliest examples of mounted combat units. Continuing developments in arms and armor eventually led to knights, dragoons, and finally to the advanced cavalry of the 18th and 19th century.