Modern horses are not indigenous- things change a bit in 13, 000 - 2 million years..
Modern horses, zebras, and asses belong to the genus Equus, the only surviving genus in a once diverse family, the Equidae. Based on fossil records, the genus appears to have originated in North America about 4 million years ago and spread to Eurasia (presumably by crossing the Bering land bridge) 2 to 3 million years ago. Following that original emigration, there were additional westward migrations to Asia and return migrations back to North America, as well as several extinctions of Equus species in North America.
The last prehistoric North American horses died out between 13,000 and 11,000 years ago, at the end of the Pleistocene, but by then Equus had spread to Asia, Europe, and Africa.
Animals that on paleontological grounds could be recognized as subspecies of the modern horse originated in North America between 1 million and 2 million years ago. When Linnaeus coined the species name, E. caballus, however, he only had the domesticated animal in mind. Its closest wild ancestor may have been the tarpan, often classified as E. ferus; there is no evidence, though, that the tarpan was a different species. In any case the domesticated horse probably did not arise at a single place and time, but was bred from several wild varieties by Eurasian herders.
Well thanks for taking the trouble to surf the net a bit and prove my point. As I said, horses are indigenous to North America, and all horses originated from North American stock.
And by the way, horses have not changed consequentially in 15,000 years other than being tampered with by man. The horse of 15,000 years ago was an Equus just as today - otherwise known as a modern horse. Once again, if you are going to use history, use all of it - not just enough to accomplish your agenda. Heck, if you want to carry it to the extreme, your Arabs are not "modern" horses in the literal sense of the word as they are a very old breed.
My overall point, and it is true, is that the perception that Arabian horses are the root of everything is completely false...Arabs have their roots in other breeds just as all horses do. Just because those breeds are no longer around is not relevant.
Arabs are not the beginning, nor are they the end. Is the glass half full or is it half empty? Is the Arab used to improve other breeds as you say, or have Arabs been refined to become faster and stronger by mixing in other breeds to develop Thoroughbreds, Quarterhorses, and others that are far superior at their given task than Arabs?