Livestock, i.e. cattle, sheep, pigs and horses, have almost as long a history with people as dogs. Sheep, for example, were domesticated between 8000 and 7500 BCE for their meat. They were covered with hair called kemp; their wool was a layer close to the skin and made up of short fibers. How did we get the spinnable long fibers sheep have now? Mutation? Or did the people then do a little selective breeding? We don’t know.
So, on the steppes, horses were –somewhat – domesticated pre-5000 BCE.
Horses originally roamed not only the Eurasian steppes but also the North American plains. Fossils of early and not so early horses have been found here in the USA. But the North American horses disappeared, no one is quite sure why. Climate change has been suggested as one possibility. But I digress.
Like sheep and cattle, horses were first used as a source of meat. Not as draft animals. And then, with the human ability to see other advantageous qualities, someone started riding. Although I can’t know for sure, I am pretty positive the first mount would be a mare. Stallions are fierce and aggressive. But gelded males and mares are more manageable, especially if they have had regular contact with people from birth. And horses, at approximately 800 pounds, are large enough to carry a rider or pull a vehicle.
A man on foot can control 200 sheep; on horseback 500. Leather and rope bits and cheek pieces begin showing up in grave goods. Horses begin taking on value and are only eaten at feasts (where the chief is showing off.}
I will add that of course, given our human propensity for war, the horse became part of the effort. Not just as a mount for a horseman but also as a team pulling war chariots. Now chariots speak to a ruling class with the necessary time and resources for the purchase of a chariot and the team of horses as well as the training necessary to use them effectively in battle. I’ve seen a few graphics of an Egyptian pharaoh driving such a vehicle with the reins looped around his hips. Since chariots were used only for war, they had their moment in history. But they didn’t come down through the millennia the way wagons did.