You're probably after a horse from America, but in Australia there's a mare called Black Caviar. 20 runs, unbeatean.
Kentucky
Apparently the most famous breed of horse is the beautiful Arab.
The most famous Quarterhorse halter horse is probably "Impressive"
i would say probably the Kentucky derby at Churchill Downs in Louisville Kentucky
it is the name of a famous race horse. A movie was made about this horse.
the Clydesdale
I know California is the most horse-populated state in the U.S.
secretariat
Secretariat/Red
Thoroughbred horse
Thoroughbreds are one famous breed from England.
That really depends on where you're from and where you live. In North America, likely the most famous breed of horse would be the American Mustang, followed by the American Quarter Horse. In the Middle East, the most famous breed would be the Arabian horse.