Which Vedic royal ritual involved the sacrifice of a horse?

The Vedic royal ritual involving the sacrifice of a horse was the Ashvamedha.

The Ashvamedha was both a religious sacrifice and a declaration of political supremacy. A consecrated stallion was allowed to wander for a year, accompanied by the king’s warriors. Rulers who challenged the horse’s passage were effectively challenging the sponsoring king; territories that did not resist were symbolically placed under his authority.

After the horse returned, an elaborate sequence of ceremonies culminated in its sacrifice and the king’s proclamation as an undisputed sovereign over the lands represented by the horse’s journey. The ritual required major resources, priests, attendants, animals, gifts, and precisely ordered rites. It is described in Vedic ritual literature and receives a prominent narrative treatment in the Mahabharata’s Ashvamedhika Parva. Vajapeya was another prestigious Vedic sacrifice, Rajasuya was a royal consecration ceremony, and Agnihotra was a recurring fire offering—not the horse sacrifice.