It is well established, but oft overlooked, fact that almost all thoroughbred horses on the British Islands trace their ancestry to a single stallion, called Darley Arabian, brought to Britain by one Thomas Darley in 1704. We know that there was one famous horse prior to that date, Byerley Turk, brought to Britain in 1686 (his influence on the modern horses is much more modest, but at least we know he existed).
The descendants of those horses are very well accounted for in breed books; even more so, best horses, including the aforementioned breed progenitors, were painted on canvas, so we know exactly what they looked like. This is not at all surprising, considering how popular horse racing was and still is.
It is also highly unlikely that people were not at all interested in breeding fast horses prior to 1686/1704 - after all, back then the stakes on "bet to win" could literally be one's life in battle.
So, the question is, what do we know about the thoroughbred development history prior to 1686 (in Britain or elsewhere; Britain just happens to be the best in keeping its history in orderly fashion)?
It is, of course, trivial fact that the term "thoroughbred" was only invented in the beginning of 18th century. My question begs to establish how the horse breeding was organized prior to the beginning of "thoroughbred" era (the term may be British, but the concept is definitely not).
People who started the "thoroughbred" lineages definitely knew the value of good racing horse. They went through great expenses to bring those horses from the Middle East, breed them as extensively as they could and even bothered to commission portraits of those horses (so it was not an idle fancy by any measure).
We know, that prior to invention of the term "thoroughbred", racing horses went by the name "palfrey". Good palfreys were as valuable as any later day thoroughbred and were used for the same exact purpose (horse back riding).
So, does anybody has any leads on studbooks or individual portraits of horses from the pre-thoroughbred time. I, personally, can not imagine that nobody bothered to keep a lineage or portrait of one of those super-expensive palfreys or destriers (such lineage would be handy for marketing purposes back then, if nothing else; a price of a single "noble" horse, as far as we know, was comparable to a price of a village, with land, peasants and all).