<P> All Equidae in North America ultimately became extinct, along with several other megafauna . The causes of this extinction have been debated . Given the suddenness of the event and because these mammals had been flourishing for millions of years previously, something quite unusual must have happened . The first main hypothesis attributes extinction to climate change . For example, in Alaska, beginning approximately 12,500 years ago, the grasses characteristic of a steppe ecosystem gave way to shrub tundra, which was covered with unpalatable plants . However, it has been proposed that the steppe - tundra vegetation transition in Beringia may have been a consequence, rather than a cause, of the extinction of megafaunal grazers . </P> <P> The other hypothesis suggests extinction was linked to overexploitation of native prey by newly arrived humans . The extinctions were roughly simultaneous with the end of the most recent glacial advance and the appearance of the big game - hunting Clovis culture . Several studies have indicated humans probably arrived in Alaska at the same time or shortly before the local extinction of horses . </P> <P> Horses returned to the Americas thousands of years later, well after domestication of the horse, beginning with Christopher Columbus in 1493 . These were Iberian horses first brought to Hispaniola and later to Panama, Mexico, Brazil, Peru, Argentina, and, in 1538, Florida . The first horses to return to the main continent were 16 specifically identified horses brought by Hernán Cortés in 1519 . Subsequent explorers, such as Coronado and De Soto brought ever - larger numbers, some from Spain and others from breeding establishments set up by the Spanish in the Caribbean . </P> <P> These domesticated horses were the ancestral stock of the group of breeds or strains known today as the Colonial Spanish Horse . They predominated through the southeast and western United States from 16th century until about 1850, when crossbreeding with larger horse breeds changed the phenotype and diluted the Spanish bloodlines . Later, some horses became strayed, lost or stolen, and proliferated into large herds of feral horses that became known as mustangs . Modern domesticated horses that retain Colonial Spanish type include the Spanish Mustang, Choctaw horse, Florida Cracker horse, and the Marsh Tacky . One somewhat obscure and largely debunked hypothesis posits that horses survived the ice age in North America . This idea mostly comes from Dakota / Lakota oral histories, which refer to straight - backed ponies, around 13 hands tall, with very powerful lungs . </P>

When were horses introduced to the united states