The horse (Equus ferus caballus) is one of two extant subspecies of Equus ferus. It is an odd-toed ungulate mammal belonging to the family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature, Eohippus, into the large, single-toed animal of today. Humans began to domesticate horses around 4000 BC, and their domestication is believed to have been widespread by 3000 BC. Horses serve in a wide variety of sport competitions and non-competitive recreational pursuits, as well as in working activities such as warfare, police work, agriculture, entertainment, and therapy. Some cultures eat horses, though in others this is taboo, whether officially or unofficially.
Horses are present in nearly every Harry Turtledove work, usually in their millennia-old OTL role of pack-animal, livestock, or warhorse. In a few fantasy stories, the unicorn (a mythical beast usually depicted as a horse with a horn on its head) exists alongside and/or in place of horses. Only in a few stories is the presence (or absence) of horses relevant to understanding how a particular world works.
In the magical world to which Hasso Pemsel found himself transported, horses existed alongside unicorns. Unlike riding a unicorn, riding a horse did not require the rider to have magical blood or any other supernatural ability.
During the Black Plague, many vampires, Jerome among them, had to feed on horse blood, due to a shortage of human flocks. Five centuries later, the remembered experience still rankled in Jerome's mind.
It was a matter of some debate within the Baron Toivo expedition as to whether the unicorns of the tropical continent were more closely related to horses or goats. Kyosti though they might even be a wholly unknown family.
Horses did not exist in the world where Georgia and Lani lived. Instead, people rode on rabbits which were the size of horses, not that a person from that world could make any sense of this comparison.
After The Change rendered all internal combustion engines and electric devices useless, horses resumed their status as the primary transportation vehicle of the world. In warfare, horses took the place of tanks. Bruce Delgado, head of the Chatsworth Lancers, named his horse Sherman after the M4 Sherman tank for this reason.
After the Russian-American War of 1967 set global civilization backward and caused the virtual extinction of the automobile, horses and other equine species such as donkeys reclaimed their position as man's primary method of transportation. It was an honor they probably could have lived without.
For most of the series, Detinan characters acknowledge only two equid species: unicorns and donkeys. However, there are a few inconsistencies where horses are referenced when unicorns would make more sense in context.
The Race's first view of Tosevites included an image of an armored man riding a horse, taken in the 12th century of the Tosev 3 Common Era. By the time the Race attempted to conquer the planet in 1942, horses were fairly obsolete, being replaced by automobiles. Ironically, the Race's invasion, by disrupting the flow of oil and other resources, caused a resurgence of the horse's importance in warfare for the duration of the conflict.