What is a semi-wild horse?

Semi-wild is a domesticated horse that has become wild again.

a semi wild horse is a fjord

A "semi-wild horse" is a Mustang. They are thus called because of their unusual history.

Horses are not native to the Americas, no horse had ever been seen in America until the Spainards and the English brought them over. Some of the horses, the brave, resourceful, and free-spirited ones, managed to escape and, their spiecies being suited to the enormous plains of the mid-west, as yet open, they began to form herds of free horses. The Mustangs roamed nomaically in the mid-west for generations, but, after the American Revolution, frontiersmen began to hunt down the mustangs for use as mounts, because they were considered the best horses in the world. There numbers dwindled as the development of the Midwest left less and less space for them to hide, and eventually they were all re-captured. Many of them killed themselves (the practice of slitting ones wrists comes from the way captured horses occansionally kill themselves rather than become essentially slaves) and the rest were reintroduced as domestic animals. Virtually no true wild mustangs exist today, although purebred mustangs are still highly valued.