Often each state will have an association that keeps track of horses and their owners. For example in TN there is the Walking Horse Owners’ Association. Thoroughbreds are kept track of in the Jockey Club.
The transfer of ownership for all registered Thoroughbreds may be reported to The Jockey Club by completing a Transfer of Ownership Form or through Interactive Registration at http://registry.jockeyclub.com.
For Florida bread horses, they are kept track of through the Florida Thoroughbred Breeders’ & Owners’ Association