The Scout is the only universally available cavalry unit. The second most widespread is the Cavalry, which is available to 14 of the 16 civilizations.
In contrast, Cataphracts and Armored Elephants are each available to only 6 civilizations.
Although no civilization has a complete Stable technology tree, the Palmyrans are only missing the Cataphract.
In contrast, the Greeks and Minoans are each missing 6 of the 9 available cavalry units.
The horses that survived the last Ice Age were relatively small animals unsuited for riding or pulling. They were hunted out of existence in the Americas and domesticated first for food on the steppes of Asia. Over many generations of selective breeding, they grew large enough to be of use other than as food. One issue that had to be resolved was how to harness them without causing choking. Humans eventually learned to ride, first from the rear, non- control position over the hips, and then from the forward position that we are familiar with today. The first evidence of horses being ridden appears in the second millenium B.C., although it is generally accepted that they were ridden earlier in Asia. The Stable represents the application of animals, primarily the horse, to warfare, first pulling chariots and then carrying warriors. Detailed records survive from Assyria and elsewhere related to the acquisition, training, equipping, and employment of horses in battle.