Hipparions were not “primitive horses”, although the picture may make them look like that! They were the major tribe (in terms of number of species/geographic distribution) in the Miocene-Recent equid subfamily Equinae. Another tribe, the Equini, became single-toed and survived to the present day, but the hipparions survived until around 1 Ma, and many species were as large as modern horses. So, they were cousins to extant equids, not more primitive relatives.