What English names with the word
In ordinary English a horse is the large solid-hoofed mammal people ride, drive, and race: the domestic species Equus caballus, and by a slightly wider use other members of the same family. Dictionaries still keep a narrower sense for an adult male, and a military sense in which horse means cavalry rather than a single animal. None of that is new. The noun has named the animal since Old English. The origin of the word horse is not a question about the creature. It is a question about which of several old names English kept, and why the trail behind that name stops being certain.
A native Germanic inheritance
Horse is not a French import and not a learned Latin coinage. Old English already had hors for the animal. Middle English wrote hors and horse. Standard historical dictionaries place the English noun in the Old English period, before the twelfth century, and treat it as inherited from Germanic rather than borrowed later. Norman French brought cheval, from Latin caballus, originally a workhorse or nag; Spanish caballo is the same stock. Those words never replaced the English form.
The Germanic family is dense and easy to see. Old Norse had hross. Old Frisian shows hors, hers, hars, and ros. Old Saxon had hross or hors. Old High German hros or ros survives as German Ross. Dutch has ros, with dialectal hors. Old Swedish hors and the Gotland form russ belong here as well, as do Icelandic hross and several Scandinavian dialect words still meaning a horse or a mare. From that set, handbooks reconstruct a Proto-Germanic neuter, most often written hrussą, with variant reconstructions hursą or harss- depending on the author. English horse is the West Germanic descendant of that noun, through Old English hors.
The morphology inside English is simple. Speakers do not take the modern word apart into a prefix and a root. What did change is how they counted. Old English used the same form for one animal and for many, a collective singular. Early Middle English also had horsen, from an old dative plural. The regular plural horses is already in Layamon around 1205; by the seventeenth century it was the usual literary form. Military English can still say a troop of horse, keeping the older collective beside the everyday horses.
Three answers for the step before Germanic
Once the comparison leaves Germanic, the evidence splits, and the honest account is that it stays split.
One proposal links the noun to a Proto-Indo-European root meaning to run, often written kers or kers- with a palatal, through a derived form meaning a runner or a vehicle. On that view the Germanic word would sit near Latin currere, to run, and more loosely near the family that later gave English car. Some etymological dictionaries present this as a live option. Others mention it only as a connection some scholars make. The Oxford English Dictionary, more cautious, stops at Germanic inheritance and calls the further etymology unknown.
A second proposal treats Proto-Germanic hrussą as a loan from a Northeastern Iranian language, often labeled Sarmatian. Comparisons offered include a reconstructed Sarmatian form wurš and Ossetian wyrs, stallion. Dirk Boutkan has favored this loan account and has also pointed to a Uralic borrowing in the same region, such as Finnish varsa, foal. The historical picture is tempting: steppe peoples, horses, and words can travel together. The linguistic picture is thinner. Sarmatian itself is poorly attested, so the match remains a hypothesis, not a demonstration.
A third suggestion, ranked weaker in the handbooks, derives the noun from a Germanic verb of falling or rushing, the family of Old English hreosan, to fall. Few accounts rest the history on that verb alone.
Germanic already had other horse-words, which matters because it shows that hrussą was never the only label. The old Indo-European name, from a root that yields Latin equus, Greek hippos, and Sanskrit asva, survived in Old English as eoh. Another Old English word, wicg, goes back to Proto-Germanic wigją and is itself of uncertain origin. A third stem, the ancestor of mare, named a horse of a particular kind. In several Indo-European languages the inherited equus-word faded. Scholars have long suspected a religious taboo on uttering the name of an animal so central to cult and war. English kept horse instead of eoh.
Later still, German and Dutch added other everyday labels that can hide the older family from a casual look. German Pferd and Dutch paard come from a palfrey path, not from hrussą. Swedish häst and Danish hest belong with a different Germanic set, the one that also gives English henchman. Those replacements show how crowded the field became. They do not cancel the native English line.
Senses that grew around the animal
The core meaning never left. From the late fourteenth century the noun also named frames and devices that suggest something you mount or straddle: a sawhorse, later a gymnastic horse. Cavalry use is old enough that horse in the plural can still mean mounted troops. Geology borrowed the word for a mass of barren rock inside a mineral vein. Informal American English later used horses for horsepower. Slang heroin is much later, attested around 1950.
The verb is already Old English: horsian meant to furnish with horses. Later English added hauling by brute force, and the informal sense of horsing around. Adjectival uses such as horse guards or horse pills (large or coarse of their kind) grew from the same noun without a new etymology.
Everyday phrases keep the animal in view: looking a gift horse in the mouth, holding one’s horses, hearing something from the horse’s mouth. They show how thoroughly the word sat in daily life. They do not explain how it was formed.
What the history actually settles
English did not keep the ancient Indo-European horse-name that Latin and Greek kept. It kept a Germanic word whose living cousins include Icelandic hross and German Ross, first recorded in English in the Old English period, and whose last step—run-root, Iranian loan, or something still unnamed—is argued rather than solved. That last gap is the useful fact. The everyday English noun is native. The pedigree behind Proto-Germanic is not.