Śatapatha Brāhmaṇa ("of a hundred paths") 13.2.2.19

13.2.2.19

atha yadāyasā itareṣām viḍvā itare paśavo viśa etadrūpaṃ yadayo viśameva tadviśā samardhayati vaitasa iṭasūna uttarato'śvasyāvadyantyānuṣṭubho vā aśva ānuṣṭubhaiṣā dikṣvāyāmevainaṃ taddiśi dadhātyatha yadvaitasa iṭasūne'psuyonirvā aśvo'psujā vetasaḥ svayaivainaṃ yonyā samardhayati

And as to why there are iron ones for the others,--the other animals, indeed, are the peasantry, and this--to wit, iron--is a form of the peasantry: he thus combines the peasantry with the peasantry. On a rattan mat (lying) north (of the Âhavanîya) they cut the portions of the horse(-flesh); for the horse is of anushtubh nature, and related to the Anushtubh is that (northern) quarter: he thus places that (horse) in its own quarter. And as to (his doing so) on a rattan mat,--the horse was produced from the womb of the waters, and the rattan springs from the water: he thus causes it to be possessed of its own (maternal) womb.