One of the rituals described in the Vedas is ashvamedha, the horse sacrifice. This was a political ritual and it concerned the king’s right to rule. First a horse, always a stallion, would be allowed to wander around freely for a year, accompanied by members of the king’s retinue. If the horse wandered off into the land of an enemy, that territory had to be occupied by the king. Any rival of the king was free to challenge the horse’s attendants to a fight. If they did, and the horse was killed, the king would lose his right to rule. If, on the other hand, the horse still was alive after a year, it was taken back to the king’s court. Here it was bathed, anointed with butter, decorated with golden ornaments and then sacrificed. Once this ritual was completed the king was considered as the undisputed ruler of all the land the horse had covered. All kings in Vedic India performed the ashvamedha, and the ritual declined only in the latter part of the Gupta period.
Central Asia, not India, is where the horse originates and the ashvamedha is one piece of evidence which locates the Indo-Europeans outside of India. Horse sacrifices have been carried out all over the Eurasian landmass — in China, Iran, Armenia, among the Greeks and the Romans, even in Ireland. To the people of the steppe, the horse was a sacred animal, and horses were often buried together with dead kings. In the Irish ritual, the king had sexual intercourse with a mare who then was killed, dismembered and cooked in a cauldron in which the king proceeded to swim and drink from the broth.
New-age Hindu spiritualists have recently tried to revive the ashavamedha ritual, but they use a statue of a horse rather than an actual animal. In other contemporary rituals, live horses are worshiped rather than killed. Apparently devotion to the horse can help you defeat enemies and clear debts. The first critics of the ashavamedha appeared already among members of the Carvaka school of philosophy in the seventh century BCE. The Carvaka philosophers were skeptics and atheists. They had no doubt that horse sacrifices were invented by “buffons, knaves and demons.”
In Our Time: “The Upanishads”
History of the World in 100 Objects, “Gold coin of Kumuragupta I”
History of Philosophy Without Any Gaps: “Vedic India: Kingdom for a Horse”