Śatapatha Brāhmaṇa ("of a hundred paths") 6.4.4.12

6.4.4.12

athaitānpaśūnāvartayanti | teṣāmajaḥ prathama etyatha rāsabho 'thāśvo 'theto yatāmaśvaḥ prathama etyatha rāsabho 'thajaḥ kṣatraṃ vā anvaśvo vaiśyaṃ ca śūdraṃ cānu rāsabho brāhmaṇamajaḥ

They then make the beasts return (to the Âhavanîya): the he-goat goes first of them, then the ass, then the horse. Now, in going away from this (Âhavanîya), the horse goes first, then the ass, then the he-goat,--for the horse corresponds to the Kshatra (nobility), the ass to the Vaisya and Sûdra, the he-goat to the Brâhmana.