The phrase "horse domestication" means taking wild horses and training them to be used by humans for different purposes, like riding or pulling carts. Full definition
The first three millennia of horse domestication thus preserved a large diversity of male lineages. (sciencedaily.com)
The data suggest that around 3500 B.C. — roughly the same time that many linguists place the origin of PIE and that archaeologists date horse domestication — Yamnaya genes replaced about 75 percent of the existing human gene pool in Europe. (scientificamerican.com)
Instead, phylogenetic analysis shows Przewalski's horses are feral, descended from the earliest - known instance of horse domestication by the Botai people of northern Kazakhstan some 5,500 years ago. (sciencedaily.com)