In ancient Roman religion, the October Horse (Latin Equus October) was an animal sacrifice to Mars carried out on October 15, coinciding with the end of the agricultural and military campaigning season. The rite took place during one of three horse-racing festivals held in honor of Mars, the others being the two Equirria on February 27 and March 14.

Two-horse chariot races (bigae) were held in the Campus Martius, the area of Rome named for Mars, after which the right-hand horse of the winning team was transfixed by a spear, then sacrificed. The horse's head (caput) and tail (cauda) were cut off and used separately in the two subsequent parts of the ceremonies: two neighborhoods staged a fight for the right to display the head, and the freshly bloodied cauda was carried to the Regia for sprinkling the sacred hearth of Rome.

Ancient references to the Equus October are scattered over more than six centuries: the earliest is that of Timaeus (3rd century BC), who linked the sacrifice to the Trojan Horse and the Romans' claim to Trojan descent, with the latest in the Calendar of Philocalus (354 AD), where it is noted as still occurring, even as Christianity was becoming the dominant religion of the Empire. Most scholars see an Etruscan influence on the early formation of the ceremonies.

The October Horse is the only instance of horse sacrifice in Roman religion; the Romans typically sacrificed animals that were a normal part of their diet. The unusual ritual of the October Horse has thus been analyzed at times in light of other Indo-European forms of horse sacrifice, such as the Vedic ashvamedha and the Irish ritual described by Giraldus Cambrensis, both of which have to do with kingship. Although the ritual battle for possession of the head may preserve an element from the early period when Rome was ruled by kings, the October Horse's collocation of agriculture and war is characteristic of the Republic. The sacred topography of the rite and the role of Mars in other equestrian festivals also suggest aspects of youth initiation and rebirth ritual. The complex or even contradictory aspects of the October Horse probably result from overlays of traditions accumulated over time.

The rite of the October Horse took place on the Ides of October, but no name is recorded for a festival on that date. The grammarian Festus describes it as follows:

The October Horse is named from the annual sacrifice to Mars in the Campus Martius during the month of October. It is the right-hand horse of the winning team in the two-horse chariot races. The customary competition for its head between the residents of the Suburra and those of the Sacra Via was no trivial affair; the latter would get to attach it to the wall of the Regia, or the former to the Mamilian Tower. Its tail was transported to the Regia with sufficient speed that the blood from it could be dripped onto the hearth for the sake of becoming part of the sacred rite (res divina).

In a separate passage, the Augustan antiquarian Verrius Flaccus adds the detail that the horse's head is adorned with bread. The Calendar of Philocalus notes that on October 15 "the Horse takes place at the Nixae," either an altar to birth deities (di nixi) or less likely an obscure landmark called the Ciconiae Nixae. According to Roman tradition, the Campus Martius had been consecrated to Mars by their ancestors as horse pasturage and an equestrian training ground for youths.

The "sacred rite" that the horse's blood became part of is usually taken to be the Parilia, a festival of rural character on April 21, which became the date on which the founding of Rome was celebrated.