Śatapatha Brāhmaṇa ("of a hundred paths") 13.3.2.2

13.3.2.2

śakvaryaḥ pṛṣṭham bhavanti anyadanyaccando'nye'nye vā atra paśava ālabhyanta uteva grāmyā utevāraṇyā yacakvaryaḥ pṛṣṭham bhavantyaśvasyaiva sarvatvāyānye paśava ālabhyante'nye'nye hi stomāḥ kriyante

The Sakvarî (verses) are the Prishtha (-stotra of the second day): there is a different metre for each (verse), for different kinds of animals, both domestic and wild ones, are immolated here on each (day). As to the Sakvarî (verses) being the Prishtha, it is for the completeness of the horse (sacrifice); and different kinds of animals are immolated on different (days), because different stomas are performed on the different (days of the Asvamedha).