Also in the Vedas
The sacrificial animal had to be all white and fast, much like the mythical Uchchaishravas. The reference to dark spots and patches suggests that a piebald or skewbald horse was also acceptable for its striking appearance. Or it had to sport a mane that appeared midnight blue. It should have been allowed to wander about for a year accompanied by a complement of four hundred of the king’s men, including a hundred royal princes. They were required to guard the consecrated animal and defend its right to stray into the territories of other kings, over whom their lord could now claim suzerainty. The sacrifice was performed on a ground soaked with the blood of the masses of different domestic animals previously slaughtered there in preparation for the final act.
Various methods seem to have been adopted to execute the horse. It would have had to be restrained by two men as it was killed by the executioner. It might also have been strangled to hasten its death. The chief queen had to perform a particularly lurid role, obligated to lie beside the dead horse under a covering in an act of uniting with the animal – symbolically, if not in the actual sense.
"You do not really die through this, nor are you harmed. You go to the gods on paths pleasant to go on . . . " The horse is told - for the victim must consent for the sacrifice to be effective.
"Let Aditi free us from sin [including the sin of killing the horse]. Let the horse with our offerings achieve sovereign power for us.’"
https://theprint.in/pageturner/excerpt/india-native-horses-disappeared-8000-bc-rig-veda-mentions-them-more-than-cow/586756/