I was recently researching to write a post about Welsh Cobs when I came across a horse law from 1540. I thought this was quite interesting so switched to writing a post on this (the Welsh Cob one will be coming soon!).
In the 16th century, King Henry VIII wanted to improve the breeds of horses present in the UK, particularly of those horses used for war. He developed the Horses Act 1540 which said all stallions under 15 hh and all mares under 13 hh were not allowed to run out on common land or run wild, therefore many of them were destroyed. Also no 2 year old colts under 11.2 hh were allowed to run out in any area with mares. Annual round ups of the commons were enforced and any stallion under the height limit was destroyed.
The law was turned over in 1566 by Queen Elizabeth on the basis that poor lands could not support the weight of these large horses. The population size of horses in the UK had been greatly decreased over the years of enforcing this law.
With regards to the Welsh Pony and Cob (which are often smaller than the required height in this act), it was said they managed to escape the wranglers on their rocky native territory. After the act was passed, the breed was crossed with a number of other breeds such as Arabs to help increase the numbers.