The place to start is from the widely recognized three founding sires, the Darley Arabian, the Byerley Turk, and the Godolphin Arabian. Every Thoroughbred you see at every track in the world can trace their sire lineage to these stallions. This is common knowledge for all turf enthusiasts. In fact, most other breeds of horses in racing forms beyond flat racing or hunt racing have these three sires in the back pages of their bloodlines. Standardbreds (trotter and pacers) all descend from Messenger, who has all of the three founding sires in his pedigree. Quarter horses were bred from Thoroughbreds of the day along with other equine breeds (including Morgan and Palomino) to form their present breeds. Many of the top show jumpers come from Thoroughbred bloodlines due to the Thoroughbred’s obvious athleticism, so the big three will be in their lineage as well.
The three founding sires were by no means the only sires at the beginning of the Thoroughbred breed. There were many other stallions used in shaping the new breed but their male lines have not endured and for all intent are now extinct as the big three influenced future generations. They became dominant as far as direct male line descent is concerned. The Darley Arabian is the direct male line ancestor of approximately 90% of all thoroughbreds while the Byerley Turk and the Godolphin Arabian make up the remaining 10% between them. No other stallion at the dawn of the Thoroughbred has a direct male line descendant currently racing.
Some of the other stallions in the early days of forming the breed did have a huge impact on the distaff side, and early sire lines, of the first few stud books. Alcock’s Arabian, D’Arcy’s White Turk, Leedes Arabian, Curwen’s Bay Barb and the Brownlow Turk are all substantial contributors. Most of the original broodmares distinguished in the original stud book were bred to these stallions.
Broodmares were generally bred to stallions of Arabian origin. The English racing horse and Irish Hobby breed developed in or around the thirteenth century, has some influence in the distaff side of early Thoroughbreds as does the Scottish Galloway, which is now extinct as a pure breed unfortunately. It is estimated that fifty percent of the original mares were bred to stallions of Arabian descent, with the remainder divided by Turk and Barb origins. Approximately one hundred broodmares were entered in the original stud book in England, with numbers depicting each one as a family surname linking direct female line descendants from these founding broodmares.
Horses back then were also somewhat smaller than what we see in today’s Thoroughbred, averaging just under 15.0 hands compared to now where the average is just above 16.0 hands. This is likely due to the fact that as shorter races became more popular through time, the need for pure speed and the selective breeding to obtain these traits in future generations, led to larger more muscular Thoroughbreds.
Grey horses foaled throughout history have acquired their coat colour from the descendants of mares that have Alcock’s Arabian, the Brownlow Turk and D’Arcy’s White Turk in their lineage, as the three founding sires where not of this colour.