Originally the horses used for racing would have been native or home breeds and from these the more athletic of the breeds would have become increasingly popular.
Certain bloodlines would have attracted attention and here in Wales none more so than the native Welsh Cob. However, there is little evidence of any association between the Welsh Cob and the Standardbreds seen racing in the UK today. Unlike breeds such as the Norfolk Trotter, which are known to have influenced the origins of the breeding of the Standardbred worldwide.
Today, the horses used for harness racing in the UK are all registered Standardbreds and many of them will have been bred in the UK. There is however a strong North American influence with the vast majority of them having been sired (fathered) by stallions of a North American breed and the dam (the mother), if not directly imported from North America, will have very strong North American bloodlines. Largely derived from Thoroughbred bloodlines, these horses have been selectively bred over many generations for their ability to pace or trot at speed.