95% of thoroughbreds linked to one superstud
By John Pickrell
6 September 2005
https://www.newscientist.com/article/dn7941-95-of-thoroughbreds-linked-to-one-superstud/
My understanding is that the horses bred at Belle Meade produced the majority of famous thoroughbreds.
https://visitbellemeade.com/equestrian/
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.
Darley Arabian. Not shocking.
Through his super great-great-grandson racer and stallion, ECLIPSE.
This is partly just by noting almost all TBs are tail-male from Eclipse.
Add the other many many horses over many generations and Eclipse is the one.