Śatapatha Brāhmaṇa ("of a hundred paths") 13.3.1

Pragâpati's eye swelled; it fell out: thence the horse was produced; and inasmuch as it swelled (asvayat), that is the origin and nature of the horse (asva). By means of the Asvamedha the gods restored it to its place; and verily he who performs the Asvamedha makes Pragâpati complete, and he (himself) becomes complete; and this, indeed, is the atonement for everything, the remedy for everything. Thereby the gods redeem all sin, yea, even the slaying of a Brahman they thereby redeem; and he who performs the Asvamedha redeems all sin, he redeems the slaying of a Brahman.

It was the left eye of Pragâpati that swelled: hence they cut off the (meat) portions from the left. side of the horse, and from the right side of other victims.

There is a rattan mat, for the horse was produced from the womb of the waters, and the rattan springs from the water: he thus brings it in connection with its own (maternal) womb.

The Katushtoma is the form of chanting (on the first day); for a bee tore out (a piece of) the horse's thigh, and by means of the Katushtoma form of chanting the gods restored it: thus when there is the Katushtoma mode of chanting, it is for the completeness of the horse. The last day is an Atirâtra with all the Stomas --with a view to his obtaining and securing everything, for an Atirâtra with all the Stomas is everything, and the Asvamedha is everything.