<P> Horses in the United States have significant popularity and status that is acknowledged by a number of observers and researchers . There are about 9.2 million horses in the country and 4.6 million citizens are involved in the horse business . In addition, there are about 33,000 feral horses that roam freely in a wild state in certain parts of the country . </P> <P> The horse evolved in the Americas, but became extinct between 8,000 and 12,000 years ago . When the Spanish arrived on the American mainland in the 16th century, they brought horses with them and re-established the animals on the continent . </P> <P> Fossils of the earliest direct ancestor to the modern horse, Eohippus have been found in the Eocene layers of North American strata, mainly in the Wind River basin in Wyoming . Fossils found at the Hagerman Fossil Beds in Idaho, called the Hagerman horse or Equus simplicidens are from the Pliocene, dating to about 3.5 million years ago (mya). Paleontologists determined the fossils represented the oldest remains of the genus Equus . The genus Equus, which includes all extant equines, was plentiful in North America and spread into the Old World by about 2.5 mya . </P>

Who re-introduced the horse to the north american plains