Śatapatha Brāhmaṇa ("of a hundred paths") 6.3.1.22

6.3.1.22

etasyāmāhutyāṃ hutāyām | agnirdevebhya udakrāmatte devā abruvanpaśurvā agniḥ paśubhirimamanvicāma sa svāya rūpāyāvirbhaviṣyatīti tam paśubhiranvaicantsa svāya rūpāyāvirabhavattasmādu haitatpaśuḥ svāya rūpāyāvirbhavati ga

This libation having been offered, Agni went away from the gods. The gods said, 'Agni is the cattle (or, an animal), let us search for him by means of the (different kinds of) cattle: he will become manifest unto his own form.' They searched for him by means of the cattle, and he became manifest to his own form: and hence even to this day the animal becomes manifest to its own form (kind), cow to cow, horse to horse, and man to man.