Using the terms “wild horse” and “Mustang” interchangeably when discussing the free-roaming equids of the American West is a common error. Not all wild horses are Mustangs, as the distinction lies in biological classification and specific legal and historical definitions. “Wild horse” is a general term, while “Mustang” is a specific designation rooted in ancestry and government protection.
Defining the Feral Horse
The horses currently roaming North America are not biologically wild animals but are classified as feral. A feral animal lives in a wild state but is descended from domesticated ancestors. This classification is necessary because the original horse species that evolved in North America became extinct approximately 10,000 years ago during the late Pleistocene epoch.
Modern horse populations trace their lineage back to domesticated horses brought to the Americas by European explorers starting in the late 15th century. These animals, primarily Equus ferus caballus, were either intentionally released or escaped from Spanish conquistadors, settlers, and later, ranchers and miners. These free-roaming populations developed the survival instincts and social structures necessary to thrive without human intervention.
They live in bands, typically consisting of a dominant stallion, several mares, and their offspring. Their physical characteristics, such as small size and hardiness, are adaptations to the harsh, open range environment. While they live in a “wild” state, their domesticated ancestry means they remain biologically feral horses.
The Origin and Legal Status of the Mustang
The term “Mustang” refers to a subset of North American feral horses with a specific historical and legal identity. The word is derived from the Spanish mesteño, meaning “strayed” or “belonging to the Mesta.” Original Mustangs were direct descendants of the Colonial Spanish horses (cabeza de ganado) brought by Spanish explorers starting in the 16th century.
These Spanish horses were prized for their endurance, and their descendants formed the foundation stock for the herds that spread across the continent. Although the genetic makeup of modern free-roaming herds has been diluted by later escaped breeds, specific isolated populations still retain a strong genetic link to their Spanish Colonial ancestors.
In the United States, “Mustang” is often used synonymously with the legal designation “wild free-roaming horses.” This status is defined by the Wild Free-Roaming Horses and Burros Act of 1971. The Act protects all unbranded and unclaimed horses and burros found on specific public lands managed by federal agencies. A feral horse is legally a “Mustang” only if it resides within a designated Herd Management Area. A horse with the same Spanish ancestry living outside these federal territories would be classified as a feral horse or stray animal, but not a federally protected Mustang.
Feral Versus Truly Wild Equids
The distinction between a feral horse and a truly wild equid is a matter of evolutionary history and domestication status. A truly wild animal is one that has never been domesticated, meaning its lineage has never been subjected to human-directed breeding. North American feral horses, including Mustangs, are members of the domestic horse species, Equus ferus caballus.
The only species of horse considered truly wild today is Przewalski’s horse (Equus ferus przewalskii), also known as the Takhi. This species, native to the steppes of Central Asia, possesses biological differences from the domestic horse, including having 66 chromosomes instead of the domestic horse’s 64. This genetic difference indicates a separate evolutionary path that diverged long before the domestication of the modern horse.
Przewalski’s horse went extinct in the wild in the 1980s but has since been successfully reintroduced into its native habitats in Mongolia from captive-bred stock. This contrast underscores the classification of all Mustangs and other free-roaming North American horses as feral, reinforcing that they are descended from once-domesticated stock.