Horse sacrifice

Horse sacrifice is the ritual killing and offering of a Horse, usually as part of a religious or cultural ritual. Horse sacrifices were common throughout Eurasia with the Domestication of the horse and continuing up until the spread of Abrahamic religions, or in some places like Mongolia, of Buddhism. The practice is rarely observed in some cultures even today.
Many ethnic religions from Indo-European speaking peoples show evidence for horse sacrifice, and comparative mythology suggests that they derive from a purported Proto-Indo-European ritual and common root, though the practice is also observed among non-Indo-European speaking peoples, especially in nomadic societies from the Eurasian steppe.
Context.
Horses are often sacrificed in a funerary context, and interred with the deceased, a practice called horse burial. There is evidence but no explicit myths from the three branches of Indo-Europeans of a major horse sacrifice ritual based on a speculated mythical union of Indo-European kingship and the horse. The Indian Aśvamedha is the clearest evidence preserved, but vestiges from Latin and Celtic traditions allow the reconstruction of a few common attributes.
Some scholars, including Edgar Polomé, regard the reconstruction of a purported common Proto-Indo-European ritual as unjustified due to the difference between the attested traditions.
Etymology.
The Gaulish personal name "Epomeduos" is from "ek'wo-medhu-" ("horse + mead"), while "aśvamedha" is either from "ek'wo-mad-dho-" ("horse + drunk") or "ek'wo-mey-dho-" ("horse + strength").
Mythology.
The reconstructed myth involves the coupling of a king with a divine mare which produced the divine twins. A related myth is that of a hero magically twinned with a horse foaled at the time of his birth (for example Cuchulainn, Pryderi), suggested to be fundamentally the same myth as that of the divine twin horsemen by the mytheme of a "mare-suckled" hero from Greek and medieval Serbian evidence, or mythical horses with human traits (Xanthos), suggesting totemic identity of the hero or king with the horse.
Comparative rituals.
Vedic (Indian).
Ashvamedha was a political ritual that was focused on the king's right to rule. The horse had to be a stallion and it would be permitted to wander for a year, accompanied by people of the king. If the horse roamed off into lands of an enemy then that territory would be taken by the king, and if the horse's attendants were killed in a fight by a challenger then the king would lose the right to rule. But if the horse stayed alive for a year then it was taken back to the king's court where it was bathed, consecrated with butter, decorated with golden ornaments and then sacrificed. After the completion of this ritual, the king would be considered as the undisputed ruler of the land which was covered by the horse.
Roman.
The Roman "Equus October" ceremony involved:
Irish.
Geraldus Cambrensis recorded a ceremony among the Irish: 
There is in a northern and remote part of Ulster, among the Kenelcunil, a certain tribe which is wont to install a king over itself by an excessively savage and abominable ritual. In the presence of all the people of this land in one place, a white mare is brought into their midst. Thereupon he who is to be elevated, not to a prince but to a beast, not to a king but to an outlaw, steps forward in beastly fashion and exhibits his bestiality. Right thereafter the mare is killed and boiled piecemeal in water, and in the same water a bath is prepared for him. He gets into the bath and eats of the flesh that is brought to him, with his people standing around and sharing it with him. He also imbibes the broth in which he is bathed, not from any vessel, nor with his hand, but only with his mouth. When this is done right according to such unrighteous ritual, his rule and sovereignty are consecrated.
The major points of comparison involve: 
Norse.
The Norse ceremony according to the description in "Hervarar saga" of the Swedish inauguration of Blot-Sweyn, the last or next to last pagan Germanic king, c. 1080: 
The "Völsa þáttr" mentions a Norse pagan ritual involving veneration of the penis of a slaughtered stallion. A freshly cut horse head was also used in setting up a nithing pole for a Norse curse.
Archaeology.
The primary archaeological context of horse sacrifice are burials, notably chariot burials, but graves with horse remains reach from the Eneolithic well into historical times. Herodotus describes the execution of horses at the burial of a Scythian king, and Iron Age kurgan graves known to contain horses number in the hundreds. There are also frequent deposition of horses in burials in Iron Age India. The custom is by no means restricted to Indo-European populations, but is continued by Turkic tribes.