Complex sentence: The Pleistocene grasslands of North America were the birthplace of the modern horse and therefore the wild horse (the only remaining species of wild/non-domesticated horse) is very much a part of the prairie ecosystem, grazing alongside bison.
Split into simpler sentences: The Pleistocene grasslands of North America were the birthplace of the modern horse and therefore the wild horse. The only remaining species of wild horse is a part of the prairie ecosystem and grazes alongside bison.