We all have heard and know about the Ashvamedha Yagya. But what will happen to the horse after Yagna? My question is about horse.
It is strangled to death, as a sacrificial animal in the yaga ritual.
The Shatapatha Brahmana describes in great detail the Vedic rituals regarding; the preparation of altars, ceremonial objects, ritual recitations, and the Soma libation, along with the symbolic attributes of every aspect of the rituals.
Following are the excerpts of the ritual of the 'Ashvamedha', as described in the Shatapatha Brahmana. [Here only those sections that relate to the original posters's question have been given].
Let me present a brief reason as given by the scriptures as to why the ritual of the 'Ashvamedha' is carried out.
Verily, the Asvamedha means royal sway: it is after royal sway that these strive who guard the horse. Those of them who reach the end become the royal sway, but those who do not reach the end are cut off from royal sway. Wherefore let him who holds royal sway perform the horse-sacrifice; for, verily, whosoever performs the horse-sacrifice, without possessing power, is poured (swept) away.--Now, were unfriendly, men to get hold of the horse, his sacrifice would be cut in twain, and he would become the poorer for it. A hundred men clad in armour guard it for the continuity and uninterrupted performance of the sacrifice; and he will not become the poorer for it; (but if it be lost) they should fetch another (horse), and sprinkle it: this is the expiation in that case."
The steps of the ritual of 'Ashvamedha' :
A horse, a hornless he-goat, and a Gomriga 4' these they bind to the central stake.
Shatapatha Brahmana/Thirteenth Kanda/13:2:2:3 to 13:2:2:12:
And further several other animals are also bound.
Read 13:2:2:3 to 13:2:2:12
Shatapatha Brahmana/Thirteenth Kanda/13:2:2:13:
Let him rather bind seventeen animals to the central stake; for seventeenfold is Pragâpati, and the Asvamedha is Pragâpati,--thus for the obtainment of the Asvamedha. And sixteen (victims he binds) to each of the other (stakes), for of sixteen parts (kalâ) consists all this (universe); all this (universe) he thus gains.
When the victims have been bound (to the stakes), the Adhvaryu takes the sprinkling-water in order to sprinkle the horse. Whilst the Sacrificer holds on to him behind, he (in sprinkling the horse) runs rapidly through the formula used at the Soma-sacrifice, and then commences the one for the Asvamedha.
From 13:2:7:11 to 13:2:7:15; these hymns describe that by the sacrifice, the horse does not die but it goes to the gods. Even Agni, Vayu and Sun were animals and they were sacrificed and now they rule world.
The following are the sections that describe the strangling of the horse:
Now the gods, when going upwards, did not know (the way to) the heavenly world, but the horse knew it: when they go upwards with the horse, it is in order to know (the way to) the heavenly world. 'A cloth, an upper cloth, and gold,' this is what they spread out for the horse: thereon they quiet (slay) it, as (is done) for no other victim; and thus they separate it from the other victims.
When they quiet a victim they kill it. Whilst it is being quieted, he (the Adhvaryu) offers (three) oblations, with (Vâg. S. XXIII, 18), 'To the breath hail! to the off-breathing hail! to the through-breathing hail!' he thereby lays the vital airs into it, and thus offering is made by him with this victim as a living one.
The following section describes the slaughtering of the horse.
The slaughtering-knife of the horse is made of gold, those of the paryaṅgyas' of copper, and those of the others of iron; for gold is (shining) light, and the Asvamedha is the royal office: he thus bestows light upon the royal office. And by means of the golden light (or, by the light of the gold), the Sacrificer also goes to the heavenly world; and he, moreover, makes it a gleam of light shining after him, for him to reach the heavenly world.
Shatapatha Brahmana/Thirteenth Kanda/13:2:2:19:
And as to why there are iron ones for the others,--the other animals, indeed, are the peasantry, and this--to wit, iron--is a form of the peasantry: he thus combines the peasantry with the peasantry. On a rattan mat (lying) north (of the Âhavanîya) they cut the portions of the horse(-flesh); for the horse is of anushtubh nature, and related to the Anushtubh is that (northern) quarter: he thus places that (horse) in its own quarter.
Verily, the horse is slaughtered for all the deities.