Four Foundations theory

The history of horse domestication has been subject to much debate, with various competing theories over time about how domestication of the horse occurred. The main point of contention was whether the domestication of the horse occurred once in a single domestication event, or that the horse was domesticated independently multiple times. The debate was resolved at the beginning of the 21st century using DNA evidence that favored a mixed model in which domestication of the stallion most likely occurred only once, while wild mares of various regions were included in local domesticated herds.

In the 20th century, various theories were postulated. One set hypothesized multiple ancestral body types of the single species Equus ferus or the original wild horse, each adapted to a given environment. Another hypothesis held that prototypes originated from a single wild species and that all different body types were entirely a result of selective breeding after domestication. Yet another proposed that the theoretical prototypes were each separate species or subspecies. These theories were all based on body types and conformation, prior to the availability of DNA for research, and have since been superseded by modern studies.

In the past, several theories were proposed about the origin of the domesticated horse and how the variety in horse breeds developed. They generally can be subdivided in two categories, single origin versus multiple origins.

The single origin theory holds that domestication occurred once, after which all breeds arose through selective breeding.

A theory associated with James Cossar Ewart in Scotland and Johann Ulrich Duerst in Germany postulated three primitive horse types, considered subspecies of Equus caballus, as ancestors of modern breeds. They were:

To these Elwyn Hartley Edwards adds a fourth, the "Tundra Horse", supposedly ancestor of the Yakut pony, and "largely unconsidered by hippologists".

A later theory associated with European scholars such as Jimmy Speed, Ruy d'Andrade, Hermann Ebhardt and Edward Skorkowski, postulated four basic body types, which were not considered to be named species. They were:

...
Wikipedia