Hay, there! Did you know that horses of all colors, shapes, and sizes once roamed the world? Some were barely larger than a small dog. By 55 million years ago, the first members of the horse family, the dog-sized Hyracotherium, were scampering through the forests that covered North America. For more than half their history, most horses remained small, forest browsers. But changing climate conditions allowed grasslands to expand, and about 20 million years ago, many new species rapidly evolved. Some—but not all—became larger and had the familiar hooves and grazing diets that we associate with horses today. Only these species survived to the present.