The horse sacrifice is a ritual that was practiced by kings in ancient India. The Sanskrit word for it is "Ashwamedha".
There is a lot of scholarly debate about what actually happened in a horse sacrifice. There are several sources of information for the ritual, which was different at different times and places.
In the Satapatha Brahmana, the horse sacrifice is described as a fertility rite involving the queen, several of the king's other wives, and a dead horse. There is a lot of trashy banter between the queens and the priests, and the queens have some kind of sex with the dead horse.
The Ramayana describes another horse sacrifice, where three hundred horses and a lot of snakes are killed. The chief queens are "united with the horse". The sacrifices are ritually burned.
The fourteenth chapter of the Mahabharata describes another horse sacrifice.
The horse sacrifice is also somehow connected to sun worship, fertility, dominion of the king over his lands, and the cult of the twin horse-headed gods, the Asvins.
What does seem clear is that the ritual cost a lot of money, for the horses and for the hundreds of priests needed to officiate at the ceremonies, which went on for weeks.