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The domesticated horse and donkey performed most of the heavy mechanical work for humans before the invention of steam and electric engines.
The horse has taken a dominant position in transport, as a beast of burden, as a draft horse-drawn transport and as the main animal for riding.
When James Watt invented the steam engine, to demonstrate the capabilities of the machine, he had to compare the power of the new device with the power of a horse. As a result, a unit of power was introduced - horsepower.
Ecological problem
Before the invention of the internal combustion engine, the main environmental problem in cities was horse manure and urine. One horse excretes an average of 15 kg of manure and 4 liters of urine per day. In large cities of the late 19th century, such as London, Paris and New York, from 200 to 500 thousand horses were employed in transport, leaving a huge amount of manure on the streets. The problem of street cleaning was only solved with the replacement of horses by cars in the first decade of the 20th century

A mare named Tambelina was recognized [11] (July 7, 2006, the city of St. Louis, Missouri) as the smallest horse in the world - her height at the withers was 44.5 cm.
The Belgian draft horse named Big Jake is recognized (January 19, 2010, Smokey Hollow Farms, Wisconsin, USA) as the largest horse in the world - her height was 210.19 cm.

The number of species of the genus horse is thus 9-12. It is remarkable that different species from different subgenera interbreed successfully with each other, producing hybrids, which in some species are for the most part incapable of reproduction, while in others they are fertile; so, crossbreeds of a domestic horse with a donkey (usually sterile), with a dzhigetai (fertile), with a dau (or quagga), with a descendant of a donkey and a zebra, with a descendant of a horse and a quagga are known; a donkey with a horse, with a zebra, with a quagga, with a cross between a zebra and a donkey, with a kulan; a kulan with a donkey, a quagga, a zebra; horse; tiger horses with a horse, a donkey, a kulan, as well as their descendants with a horse and a donkey. Some of these hybrids are of great practical importance due to their excellent qualities.

In the case of division into three subgenera, the so-called “tiger horses” (Hippotigris), covered with numerous dark and light stripes, are distinguished from the donkey subgenus. The subgenus horse actually includes the domestic horse, tarpan and Przewalski's horse; to the subgenus donkey - domestic donkey, onager (E. onager), kulan, or dzhigetai (E. hemionus), and separated by some into a special species of kiang (E. kiang) and African donkey (E. taeniopus); according to others, African donkeys make up two varieties of the common one: E. asinus v. africanus and v. somalicus; quagga (E. quagga), Burchell's zebra (E. burchelii), and mountain zebra (E. zebra) belong to the subgenus of tiger horses, and Chapman's zebra (E. chapmanii), close to Burchell's zebra, and Grevy's zebra, considered by some to be special species (E. grevyi), close to a real zebra.

Based on the presence of calloused bumps (“chestnuts”) on both pairs of legs or only on the front, the degree of hair development on the tail and color, a few horse species are divided into two or three subgenera.
In the first case, the subgenus horse proper (Equus) is distinguished, with bumps on both pairs of legs and strongly developed hair on the tail (from the base or from half of the tail), and the subgenus donkey (Asinus), with bumps only on the front legs and a tail covered with long hair only at the end (however, in some forms related here - quagga and dau - it is covered with long hair almost from the base).

The genus Parahippus originated 23 million years ago. This beast had more adapted teeth and only three hoofed fingers. The descendants of the paragippus were representatives of the genus Merychippus (merikgippus), reaching up to one meter at the withers. Their skull already resembled the skull of modern horses, and the volume of the brain increased significantly. The genus Merychippus (merikgippus) has split into several evolutionary lines, some of which have become extinct. After an intermediate link called Dinohippus, about 4.4-4.5 million years ago, according to geneticists, the Equus line appeared, which gave rise to all modern horses, zebras and donkeys [8]. About 2.5 million years ago, representatives of the genus Equus crossed from America through Beringia to Eurasia. In the Middle East, Equus became donkeys, onagers and horses themselves (Equus caballus), in Central Africa - zebras[9].

The line leading to modern domesticated horses (DOM2) branches off, according to genomic data, from the line of the Przewalski horse 43.8 thousand years old. N., and from the line of the Lena horse (Equus lensensis) - 118.6 thousand liters. n. The extinct line Equus lenensis spread to southern Siberia. After analyzing the principal component plot (PCA), phylogenetic and f3-outgroup clustering, it turned out that the sample MerzlyYar_Rus45_23789 from the Republic of Tyva (Merzly Yar tract in the west of the Todzha depression in the upper reaches of the Bolshoy Yenisei River (locally called Biy Khem)) with an age of 24,000 years. n. carries extremely divergent mitochondrial DNA found only on the New Siberian Islands around 33,200 years ago

About 50 million years ago, the genus Orohippus (orohippus) stood out from the Hyracotherium. First of all, the limbs and teeth changed, the orohippus preferred more solid plant foods. Three million years later, the orohippus evolved into the new genus Epihippus (epigippus). Meanwhile, the climate has changed significantly, in North America it has become drier, forests have shrunk, giving way to open savannahs. This was a key event in the evolution of horses, as they were forced to adapt to new conditions. They became larger, which allowed them to move faster, and also completely switched to grass food.

The evolution of horses is well-documented with fossils showing how a small, dog-sized forest animal, the Propaleotherium, which had fingers instead of hooves, developed over 50 million years into large hoofed inhabitants of open spaces. The main speciation took place within North America, from where they penetrated into Eurasia through the existing natural bridge, when this was possible in the absence of the Bering Strait and before the formation of a powerful ice sheet.
The evolution of horses began in the Eocene, about 55 million years ago. At that time, there was hyracotherium - the ancestor of all current horses, a small animal with a shoulder height of only 20 cm, chewing leaves and fruits with its small teeth. Hyracotherium did not at all resemble today's horses. This animal had an arched back, a short neck and muzzle, short legs and a long tail. It was a stocky creature about the size of a fox, with four-fingered limbs. The skull and brain volume were relatively small.

Kulans are divided into a number of subspecies, which some zoologists consider independent species. Depending on the point of view, there are up to seven types of kulans. Some zoologists prefer to divide plains zebras into Burchell's zebras (Equus burchellii) and quaggas (Equus quagga quagga). The three species of zebras listed above are sometimes combined into a common subgenus of zebra, which, however, is conditional, since the relationship between these three species is no closer than in relation to other species.

The taxonomy of horses is controversial[2]. According to the most common taxonomy method, the genus of horses consists of several extinct and 9 modern species:
Wild horse (Equus ferus)
Domestic horse (Equus caballus), see also horse breeds
Kiang (Equus kiang)
Kulan (Equus hemionus)
Wild donkey (Equus africanus)
Domestic donkey (Equus asinus)
Mountain zebra (Equus zebra)
Desert zebra (Equus grevyi)
Burchell's zebra (Equus quagga)
† Western horse - a species that lived in North America and became extinct at the end of the Pleistocene
† Stenon's horse - existed in the Pliocene - the beginning of the Pleistocene of Eurasia
† Equus neogeus is a South American species extinct at the end of the Pleistocene, belonging to the subgenus Amerhippus[en] of the genus Horse[3][4][5].
There are some diverging opinions about this classification. The donkey and the kulan are sometimes separated into a separate genus Asinus, arguing that these two species separated from the rest in the Pliocene. The Przewalski's horse and the domestic horse are generally considered to be two different species due to their different numbers of chromosomes. Sometimes the former are still considered a domestic horse. According to paleogenetics, Przewalski's horses are feral descendants of Botai horses

Horses (lat. Equus) is the only modern genus of the horse family (Equidae) of the equine order.
A cladistic analysis based on cranial and postcranial elements (30 taxa, 129 characters) supports the monophyly of the genus Equus and denies the recognition of the genera Plesippus and Allohippus, supports the origin of the desert zebra (Equus grevyi) and members of the zebra/donkey clade from Stenon's European horses. Recent results of genome studies confirm the monophyly of the genus Equus and point to its origin about 4.0-4.5 million years ago.