Four Foundations theory

The four foundations theory was a hypothesis that, prior to domestication of the horse, there were four to seven landrace body types of the single species equus ferus or the original wild horse, each adapted to a given environment. ^[1] Another hypothesis held that the four prototypes originated from a single wild species and that all different body types were entirely a result of selective breeding after domestication.^[2] Yet another theory suggested that the four prototypes were each separate species or subspecies.^[1].^[2]
This classification was based on body types and conformation, prior to the availability of DNA for research, and has since been superseded by modern studies. Modern genetic evidence now points at a single domestication event for a limited number of stallions, combined with repeated restocking of wild mares into domesticated herds.^[3] This suggests that different body types might be a combination of both selective breeding and semi-feral landrace traits.
However developed, under the four foundations theory, all types and breeds of modern domesticated horses are thought to have developed from the following base prototypes:

• The "Warmblood subspecies" or "Forest Horse" also called the Diluvial Horse, (once proposed as Equus ferus silvaticus and hypothesized to have evolved into a later variety proposed as Equus ferus germanicus). This prototype may have contributed to the development of the warmblood horses of northern Europe, as well as older "heavy horses" such as the Ardennais.^[4][1]
• The "Draft" subspecies, a small, sturdy, heavyset animal with a heavy hair coat, arising in northern Europe, adapted to cold, damp climates, somewhat resembling today's Fjord horse and even the Shetland pony.^[1]
• The "Oriental" subspecies, (once proposed as Equus agilis) a taller, slim, refined and agile animal arising in western Asia, adapted to hot, dry climates, thought to be the progenitor of the modern Arabian horse and Akhal-Teke.^[1][2]
• The "Tarpan" subspecies: dun-colored, sturdy animal, the size of a large pony, adapted to the cold, dry climates of northern Asia, proposed as a predecessor to the Tarpan and Przewalski's Horse as well as the domesticated Mongolian horse.^[1]

The true Tarpan, however, was a wild horse subspecies, equus ferus ferus, that lived into the late 19th century before becoming extinct. It was never able to be domesticated. The original domesticated horse is widely believed to have resembled the Tarpan, though there is no current means by which to verify this hypothesis.