<P> The earliest known member of the family Equidae was the Hyracotherium, which lived between 45 and 55 million years ago, during the Eocene period . It had 4 toes on each front foot, and 3 toes on each back foot . The extra toe on the front feet soon disappeared with the Mesohippus, which lived 32 to 37 million years ago . Over time, the extra side toes shrank in size until they vanished . All that remains of them in modern horses is a set of small vestigial bones on the leg below the knee, known informally as splint bones . Their legs also lengthened as their toes disappeared until they were a hooved animal capable of running at great speed . By about 5 million years ago, the modern Equus had evolved . Equid teeth also evolved from browsing on soft, tropical plants to adapt to browsing of drier plant material, then to grazing of tougher plains grasses . Thus proto - horses changed from leaf - eating forest - dwellers to grass - eating inhabitants of semi-arid regions worldwide, including the steppes of Eurasia and the Great Plains of North America . </P> <P> By about 15,000 years ago, Equus ferus was a widespread holarctic species . Horse bones from this time period, the late Pleistocene, are found in Europe, Eurasia, Beringia, and North America . Yet between 10,000 and 7,600 years ago, the horse became extinct in North America and rare elsewhere . The reasons for this extinction are not fully known, but one theory notes that extinction in North America paralleled human arrival . Another theory points to climate change, noting that approximately 12,500 years ago, the grasses characteristic of a steppe ecosystem gave way to shrub tundra, which was covered with unpalatable plants . </P> <P> A truly wild horse is a species or subspecies with no ancestors that were ever domesticated . Therefore, most "wild" horses today are actually feral horses, animals that escaped or were turned loose from domestic herds and the descendants of those animals . Only two never - domesticated subspecies, the Tarpan and the Przewalski's Horse, survived into recorded history and only the latter survives today . </P> <P> The Przewalski's horse (Equus ferus przewalskii), named after the Russian explorer Nikolai Przhevalsky, is a rare Asian animal . It is also known as the Mongolian wild horse; Mongolian people know it as the taki, and the Kyrgyz people call it a kirtag . The subspecies was presumed extinct in the wild between 1969 and 1992, while a small breeding population survived in zoos around the world . In 1992, it was reestablished in the wild due to the conservation efforts of numerous zoos . Today, a small wild breeding population exists in Mongolia . There are additional animals still maintained at zoos throughout the world . </P>

Facts about the life cycle of a horse