Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.3.9

1.6.3.9

sa yadvartamānaḥ samabhavat | tasmādvṛtro 'tha yadapātsamabhavattasmādahistaṃ danuśca danāyūśca māteva ca piteva ca parijagṛhatustasmāddānava ityāhuḥ

And since it so developed whilst rolling onwards (vrit), it became Vritra; and since he sprang forth footless, therefore he was a serpent. Danu and Danâyû received him like mother and father, whence they call him Dânava.