There is a long history of horse racing, there are records of flat racing existed in 1174. There were three important Arabian horses imported in the 17th and 18th century, these were the Darley Arabian, the Godolphin Arabian and the Byerly Turk. All modern Thoroughbreds are descended in the male line from one of these three horses. There are fifty female lines still in existence. There are problems with this however, as coming from such a small number of horses they all have quite similar genetics. This limits the amount you can select the horses when breeding and may mean they could all be susceptible to certain diseases.
The Darley Arabian
The Godolphin Arabian
The Byerly Turk
(I do not own any of these images)
I know these images are paintings and not photos but these horses do look very different from the Thoroughbreds we have today. Probably because they are Arabs will have been crossed with other horses. And surely their heads can't have been that small in real life!
Most Thoroughbreds are descendants from the Darley Arabian. This is called the "Eclipse" line, named after the Darley Arabians grand-sire, Eclipse. It is thought that 95% of Thoroughbreds could come from this line.
The General Stud Book is the official record of the thoroughbred breeding industry and is made by Weatherbys. They reserve the right to decide what horses can be admitted to, excluded or removed from its pages. No horse is allowed to run unless it has been registered. And all foals must be registered within four months of birth.
So if you own a Thoroughbred they are likely to be a distant relative to one of these three horses!