The horse in Bhagvan Sri Ram’s Ashwamedha Yajna (Horse Sacrifice) was specifically selected on the advice of Sage Agastya. The story of the horse that was selected is interesting.
The horse of Ashwamedha Yajna should have pointed ears, the face should be red in color and the tail should be yellow in color. It should gallop like wind.
Sri Ram and Agastya Rishi went to the stable and selected a horse as per the requirement.
This horse then travelled around several countries and was followed by an army headed by Shatrughna, Hanuman, Sugriva and Jambhavan.
Finally the horse completed its journey and returned to the Ashwamedha Yajna area on the banks of Sarayu River in Ayodhya.
As part of the ritual, Sri Ram carried water from Sarayu River to bathe the horse. He then poured the water on the horse, prayed for the welfare of the world, and touched the horse.
When Sri Ram touched the horse, it turned into a pious man.
The pious man then told his story.
He was cursed by Sage Durvasa to become a horse.
The sage had then told the man that he would be relieved of the curse when he will become the horse of Ashwamedha Yajna and when Sri Ram touches it.