Śatapatha Brāhmaṇa ("of a hundred paths") 13.4.2.3

13.4.2.3

tadu hovāca bhāllabeyo dvirūpa evaiṣo'śvaḥ syātkṛṣṇasāraṃgaḥ prajāpatervā eṣo'kṣṇaḥ samabhavaddvirūpaṃ vā idaṃ cakṣuḥ śuklaṃ caiva kṛṣṇaṃ ca tadenaṃ svena rūpeṇa samardhayatīti

As to this, Bhâllaveya, however, said, 'That horse should be of two colours, black-spotted, for that (horse) was produced from Pragâpati's eye, and this eye is of two colours, white and black: he thus endows it with its own colour.'