In June of 1871, William Giles Harding and his son, John Harding Jr., traveled to Glen Flora Farm in Illinois and purchased a stallion named Bonnie Scotland for $450. Foaled in England in 1853, the bay horse was by Iago and the great broodmare Queen Mary. Bonnie Scotland was the greatest sire to ever stand at Belle Meade and his descendants represent more than two-thirds of all Kentucky Derby winners to date, including 11 Triple Crown Winners. Bonnie Scotland died in 1880, the same year he was named the leading sire in American racing.
Most of the Derby horses every year descend from a stallion born in the late 1950s or early 1960s, for that matter.
Ooh, couldn’t find that, found this instead:
Almost All Modern Horses Descended From A Few Oriental Stallions
GrrlScientist
Senior Contributor
Evolutionary & behavioural ecologist, ornithologist & science writer
https://www.forbes.com/sites/grrlscientist/2017/07/10/almost-all-modern-horses-descended-from-a-few-oriental-stallions/?sh=34fd261e16c5