I don't know about flat racing. I know that trot racing started out as races between town contestants in New England with owners and farmers using their best carriage horses as their racing animals. Many good trotters and pacers trace thier lines back to Morgan horses.

One interesting bit about flat racing I do know. African American jockies dominated the sport until the late 19th early 20th century here in the U.S. They were very well paid, even for the time. Many were millionaires at a time when other areas in sports were closed to them. I'm sorry to see that this bit of their history seems to be little known now.

I'd say try the web site for the Kentucky Horse Park. Perhaps you can pick up a bit more information there.