The Byerley Turk (c. 1680 - c. 1706), also spelled Byerly Turk, was the earliest of three stallions that were the founders of the modern Thoroughbred horse racing bloodstock (the other two are the Godolphin Arabian and the Darley Arabian).
The biographical details of the stallion are the subject of much speculation. The entry in the General Stud Book simply states: "BYERLY TURK, was Captain Byerly's [sic] charger in Ireland, in King William's wars (1689, &c.)." As for his earlier history, the most popular theory is that the horse was captured at the Battle of Buda (1686) along with the Lister Turk, who was brought to England by the Duke of Berwick. Other sources speculate he was one of three Turkish stallions captured at the Battle of Vienna. It is even possible he was bred in England from previously imported stock. He was definitely the war horse of Captain Robert Byerley, who was dispatched to Ireland in 1689 during King William's War and saw further military service in the Battle of the Boyne. According to early records, Captain Byerley was nearly captured while reconnoitering the enemy, "owing his safety to the superior speed of his horse"
As a general rule, the spelling of a name registered with the Jockey Club is considered definitive, even if it is an obvious error. However, the original edition of the General Stud Book was compiled nearly a century after the fact (in 1791) and contains several errors that have been subsequently corrected. Most sources consider the correct spelling of the horse's name to follow the correct spelling of the owner's name, Byerley.
The Byerley Turk was a dark brown or black horse of unknown breeding, but described in historic accounts as an Arabian. At the time, Turkish horses were described as descended from "those of Arabia or Persia", but stated that they were longer in the body and of a larger size. He was described as a horse of elegance, courage and speed. Many of his offspring were also noted to have been either bay or black.