Controversy surrounds the sharing of land and resources by mustangs with the livestock of the ranching industry, and also with the methods by which the BLM manages their population numbers.

[12] The mustang of the modern west has several different breeding populations today which are genetically isolated from one another and thus have distinct traits traceable to particular herds.

However, supporters of the mustang argue that the animals are merely small due to their harsh living conditions and that natural selection has eliminated many traits that lead to weakness or inferiority.

[31] One hypothesis held that horse populations north of Mexico originated in the mid-1500s with the expeditions of Narváez, de Soto or Coronado, but it has been refuted.

Horses replaced the dog as a pack animal and changed Native cultures in terms of warfare, trade, and even diet—the ability to run down bison allowed some people to abandon agriculture for hunting from horseback.

[38] Some wandered off because the Spanish generally did not keep them in fenced enclosures,[39] and Native people in the area captured some of these estrays.

[42] The Pueblo Revolt of 1680 also resulted in large numbers of horses coming into the hands of Native people, the largest one-time influx in history.

[46] The Nez Perce people in particular became master horse breeders, and developed one of the first distinctly American breeds, the Appaloosa.

Most other tribes did not practice extensive amounts of selective breeding, though they sought out desirable horses through acquisition and quickly culled those with undesirable traits.

[50] However, due to the barriers presented by mountain ranges and deserts, the California population did not significantly influence horse numbers elsewhere at the time.

[54] West-central Texas, between the Rio Grande and Palo Duro Canyon, was said to have the most concentrated population of feral horses in the Americas.

[46] Throughout the west, horses escaped human control and formed feral herds, and by the late 1700s, the largest numbers were found in what today are the states of Texas, Oklahoma, Colorado, and New Mexico.

[57][58] In 1959, geographer Tom L. McKnight[k] suggested that the population peaked in the late 1700s or early 1800s, and the "best guesses apparently lie between two and five million".

[62] In 1839, the numbers of mustangs in Texas had been augmented by animals abandoned by Mexican settlers who had been ordered to leave the Nueces Strip.

[82] Abuses linked to certain capture methods, including hunting from airplanes and poisoning water holes, led to the first federal free-roaming horse protection law in 1959.

[91] The BLM has established an Appropriate Management Level (AML) for each HMA, totaling 26,690 bureau-wide,[92][93][94] but the on-range mustang population in August 2017 was estimated to have grown to over 72,000 horses,[95] expanding to 88,090 in 2019.

[92] More than half of all free-roaming mustangs in North America are found in Nevada (which features the horses on its State Quarter), with other significant populations in California, Oregon, Utah, Montana, and Wyoming.

Multiple factors that included changing climate and the impact of newly arrived human hunters may have been to blame.

[103] Thus, before the Columbian Exchange, the youngest physical evidence (macrofossils-generally bones or teeth) for the survival of Equids in the Americas dates between ≈10,500 and 7,600 years before present.

[104] Due in part to the prehistory of the horse, there is controversy as to the role mustangs have in the ecosystem as well as their rank in the prioritized use of public lands, particularly in relation to livestock.

[107][108] Thus, this debate centers in part on the question of whether horses developed an ecomorphotype adapted to the ecosystem as it changed in the intervening 10,000 years.

It also stated that the distinction between native or non-native was not the issue, but rather the "priority that BLM gives to free-ranging horses and burros on federal lands, relative to other uses".

[113] Advocates assert that most current mustang herds live in arid areas which cattle cannot fully utilize due to the lack of water sources.

[119] Difficulty arises because mustang herd sizes can multiply rapidly, increasing up to and possibly by over 20% every year, so population control presents a challenge.

[124] Control of the population to within AML is achieved through a capture program, although there are no specific guidelines or techniques used to round up mustangs.

[125] The BLM allows the use of trucks, ATVs, helicopters, and firearms to chase the horses into holding pens or "traps".

"Bait" traps are another common way mustangs are corralled, usually with hay or water being left in a camouflaged pen while varying types of trigger systems close gates behind the horses.

[95] On December 8, 2004, a rider amending the Wild and Free Roaming Horse and Burro Act was attached to an appropriation bill before the Congress by former Senator Conrad Burns.

[56] In 2017, the first Trump administration began pushing Congress to remove barriers to implementing both the option to euthanize and sell excess horses.

Captured horses kept in sanctuaries are also marked on the left hip with four inch-high Arabic numerals that are also the last four digits of the freeze brand on the neck.