Śatapatha Brāhmaṇa ("of a hundred paths") 6.5.1.4

6.5.1.4

athājalomaiḥ saṃsṛjati | sthemne nveva yadvevājalomairetadvā enaṃ devāḥ paśubhyo 'dhi samabharaṃstathaivainamayametatpaśubhyo 'dhi sambharati tadyadajalomairevāje hi sarveṣām paśūnāṃ rūpamatha yalloma loma hi rūpam

He then mixes it with the goat's hair, just for the sake of firmness. And as to why with goat's hair,--the gods then collected him (Agni) from out of the cattle, and in like manner does this one now collect him from out of the cattle. And as to why with goat's hair, it is because in the he-goat (is contained) the form of all cattle; and as to its being hair, form is hair.