From NKAA, Notable Kentucky African Americans Database (main entry)
Walker, William, Sr. "Billy"
(born: 1860 - died: 1933)William Walker, Sr., born in Versailles, KY, rode Baden-Baden to victory at the Kentucky Derby in 1877. Walker was born a slave on the Bosque Bonita Farm in Woodford County, KY. He won his first race in Lexington, KY, at the age of 11. He became one of the leading riders at Churchill Downs between 1875-1878. After retiring in 1896, he became a trainer as well as adviser to horse breeder John Madden.
Walker is buried in the Louisville Cemetery; his grave was unmarked until Churchill Downs placed a headstone on it in 1996.
For more see Roadside History, by M. P. Hay, D. Wells and T. H. Appleton; and "Once noted jockey dies," The Daily Mail, 9/21/1933, p. 10.