system
Given the following post, respond as if you're commenting on Reddit:
human
How did the physiology of the horse change once we started riding them?
gpt
It is likely that horses/ponys were domesticated multiple times throughout human history. So the biological variances between the species started out as regional differences.

One interesting historical example of this was seen with the Mongol conquest of Asia. Mongols are known as the greatest horsemen in history. However, it is interesting to see in the secret histories that they were extremely impressed by the quality of Middle-Eastern and Eastern Europen horses. They historically rode small ponies into battle. But as they conquered, the quality of their mounts increased dramatically as they gained access to new gene pools and better horses.

But otherwise, the phenotypic equestrian traits are manipulated like any other domestic animal. In the same way that selective breeding has produced the hundreds of varieties of dogs we see today. Horses are bred for a purpose. Quarter horses and draft horses are the same species. But years of selective breeding has molded horses into the most useful forms. It's all about purposeful breeding for selection of desired characteristics.

However, as with dogs, genetically they are still the same species. And the very obvious phenotypic differences between breeds are not really as obvious at the genetic level. That is to say a German Shepherd and a Chihuahua are 99.9% the same genetically. They can still interbreed, but you would never guess that just by looking at them.