Modern horses are not native to South America, though the history of the horse family (Equidae) on the continent is complex. The modern domesticated horse (Equus caballus) arrived recently with European explorers and is classified as an introduced species. However, South America hosted distinct native equine populations for millions of years before they disappeared, creating a long period when the continent was entirely horse-free. This ancient history, followed by reintroduction, frames the biological status of horses found in the region today.
The Extinct South American Equids
South America hosted native, horse-like species beginning roughly 2.5 million years ago during the Late Pliocene epoch. These equids arrived from North America via the Isthmus of Panama as part of a massive faunal exchange. Two primary lineages established themselves across the grasslands and high plains: the native Equus species, such as Equus (Amerhippus) neogeus, and the unique genus Hippidion.
Hippidion, often called the “little horse,” was distinct from modern horses, featuring a noticeably domed nasal bone structure and deep nasoincisual notches. This specialized anatomy may have been an adaptation to the cold, dry environments of the Pleistocene. Fossil evidence shows these native equids ranged widely across the continent, with Hippidion extending into Patagonia, while E. neogeus was common in regions like the Pampas and El Chaco.
All native horses vanished during the Quaternary extinction event, a mass die-off that occurred between 12,000 and 10,000 years ago. This extinction eliminated approximately 83% of the large mammal genera on the continent, including all members of the Equidae family. The disappearance of the native equids was likely caused by a combination of factors. These included rapid climate change at the end of the last Ice Age and the hunting activities of newly arrived human populations. Following this event, no horses existed in South America for nearly ten millennia until the arrival of Europeans.
The Reintroduction of Equus caballus
The long absence of horses in the Americas ended abruptly with the arrival of Spanish explorers in the early 16th century. These expeditions brought domesticated horses (Equus caballus) from Europe, primarily from the Iberian Peninsula, as part of the Columbian Exchange. The animals were transported as logistics and cavalry assets to aid in the conquest of vast territories, such as the Incan Empire and the Rio de la Plata region.
Horses offered a tremendous logistical and military advantage, revolutionizing travel, warfare, and hunting for Europeans and Indigenous groups who acquired them. The introduction of the horse quickly reshaped the cultural and economic landscape of the continent. They were central to military campaigns, such as those led by Francisco Pizarro in the 1530s, where mounted cavalry often proved decisive.
Massive feral populations began to emerge when domesticated horses escaped from Spanish settlements or were intentionally released. The expansive, nutrient-rich grasslands of the Pampas in Argentina and the open steppes of Patagonia provided ideal conditions for these animals to thrive without natural predators. These rapidly multiplying herds formed the basis of the large, free-ranging populations seen today, establishing themselves as a permanent fixture in the South American landscape.
Ecological Status of Modern Feral Populations
Despite their presence for several centuries, modern free-roaming herds of Equus caballus are classified as non-native, introduced, or exotic species. These animals are technically considered feral, meaning they are the descendants of domesticated stock that have returned to a wild state. This classification is based on their origin outside of the continent’s post-Pleistocene ecosystem.
The large population sizes of these feral horses lead to significant environmental impacts on local ecosystems. They compete directly with native grazers for resources. Their grazing style, which involves cutting vegetation very close to the soil surface, can damage plants and inhibit regrowth. This intense grazing pressure often results in changes to vegetation composition, sometimes promoting the spread of non-native plant species.
The physical presence and activities of the horses also alter the habitat itself. Their movement can lead to soil compaction, and their concentrated use of water sources can exclude native wildlife. Consequently, in many protected areas, such as those in Argentina, feral horses are viewed as an invasive species that challenges biodiversity conservation efforts.