- "No hour of life is wasted that is spent in the saddle."
– Winston Churchill
- "A man on a horse is spiritually as well as physically bigger than a man on foot."
– John Steinbeck
When man realizes that he can ride on a horse (instead of just using it as a work engine), he becomes a true monster out of legend. A whole new class of weapons is born, - fast, maneuverable and difficult to kill, which will create headaches for generals for millennia to come. This class is also supported by its own building in the Encampment - the Stable.
Horseback Riding is an important early Classical Era gateway tech - it is directly or indirectly required to advance towards most techs in the lower part of the tree, so you should eventually research it. If you are a militaristic civ, you should do it as early as possible, because this tech will unlock, or lead to unlocking of most units of the Medieval Era (besides, of course, the Horseman, which is an incredibly powerful unit in the early game!). And even if you're not particularly battle-oriented, you will need it for Apprenticeship, which is one of the most important techs of the early game.
You can beeline easily Horseback Riding by researching Animal Husbandry -> Archery -> Horseback Riding (besides, unlocking the respective Eurecas is relatively easy). Having Horsemen will give a civ great advantage over others possessing even the more powerful units of the Ancient Era.
There is archaeological evidence that around 4000 BC humans had used bits on their horses in the basins of the Dnieper and Don rivers; skeletons of horses found in the region shows signs that the horses chomped on bits. Thus, horseback riding. It is thought that the Scythians of the steppes may well have been the first to develop the stirrup and the saddle, although the historical argument is as yet unconvincing. (And ancient Greek historians also claim that the Scythians were the first to geld stallions, hence making them more docile and controllable ... but that's another story.)
However, horses in the Bronze Age were relatively small by modern standards, some argue too small to be ridden comfortably or for any distance and more likely to have pulled wagons or chariots. On the other hand, Fell ponies – descended from Roman cavalry mounts – can easily carry a full-grown adult, and so it is likely that there was cavalry around long before the Greeks and Romans took to the battlefield (although this light cavalry was hardly a dominate factor in victory, mostly being used to slaughter the defeated foot soldiers trying to flee).
By the Middle Ages, however, heavily armored mounted knights dominated warfare in Europe. And in the Far East, Japanese samurai fought from horseback for centuries, although mostly as archers. Indeed, horseback riding archers were the most common type of cavalry in Central Asia for a millennium. The Arabians, astride their magnificent stallions, spread Islam across the Middle East, North Africa, and into India and Europe. Horseback riding quite literally shaped history.
Save in the Americas, where the native horse breeds had died out. Horseback riding was brought back to North America by European explorers and conquistadors, commencing with Columbus's 1493 voyage. Soon enough, the Plains Indians would be combating the white settlers from horseback.