Europe already had herds of ponies and small horses when it was part of the Roman Empire. Cold regions had smaller ponies with thicker hair. Cool, wet regions tended to raise horses with larger bones and heavy muscle. Hot, dry regions favored horses with thin, dense bones, light bodies, and short hair.
The average horse in medieval Europe would be considered small today. Horses are measured to the top of a shoulder, called the withers. One hand equals four inches. A typical modern horse is about 15 hands high, while a modern pony is typically between 12 and 13 hands. But most medieval horses were more nearly pony sized. Their “great horse” for war was the size of an average modern horse.
Providing horses for war and civil use was a constant endeavor. A mare can produce no more than one foal per year, and often less. There was growing demand for horses as both warfare and agriculture in Europe came to depend on them. The old methods of keeping a herd of horses to breed undirected were not good enough, and, by the close of the Middle Ages, Europe had many aggressive breeding programs and an international market.