Brihadaranyaka Upanishad 1.4.4

1.4.4

sā heyamīkṣāṃ cakre kathaṃ nu mātmana eva janayitvā saṃbhavati hanta tiro'sānīti sā gaurabhavadṛṣabha itarastām̐ samevābhavattato gāvo'jāyanta vaḍavetarābhavadaśvavṛṣa itaro gardabhītarā gardabha itarastām̐ samevābhavattata ekaśaphamajāyatājetarābhavadvasta itaro'viritarā meṣa itarastām̐ samevābhavattato'jāvayo'jāyantaivameva yadidaṃ kiṃca mithunamā pipīlikābhyastatsarvamasṛjata

She thought, 'How can he be united with me after producing me from himself? Well, let me hide myself.' She became a cow, the other became a bull and was united with her; from that cows were born. The one became a mare, the other a stallion; the one became a she-ass, the other became a he-ass and was united with her; from that one-hoofed animals were born. The one became a she-goat, the other a he-goat; the one became a ewe, the other became a ram and was united with her; from that goats and sheep were born. Thus did he project everything that exists in pairs, down to the ants.