The earliest extant version of this legend dating from the Nara period (surviving in an extract quoted in a medieval work) has Mutō revealing himself to be Susanoo, suggesting that the two deities were already being conflated during the 8th century, if not earlier.

[6][7] Sources that equate Gozu Tennō with Susanoo first appear during the Kamakura period (1185–1333), although one theory supposes that these three gods and various other disease-related deities were already loosely coalesced around the 9th century, probably around the year 877 when a major epidemic swept through Japan.

Mutō's name for instance is believed to derive from the Korean word mudang, "shamaness",[11] while a story recorded in the Nihon Shoki (720 CE) claims that Susanoo, after his banishment from heaven, came down to a place called 'Soshimori' in the land of Silla and from there crossed the sea to Japan.

[7] In later forms of this story featuring Gozu Tennō, the princess is given the name Harisaijo (頗梨采女 or 波利采女, also known as 'Harisainyo' or 'Barisainyo') or 'Harisai Tennyo' (頗梨采天女) and is identified as the third daughter of the dragon (nāga) king Sāgara.

These deities were amalgamated with both the Hasshōshin (八将神, "Eight Divine Generals"), the guardians of the eight directions in Onmyōdō,[32][33][34] and eight of Susanoo's sons and daughters (Yashimajinumi, I(so)takeru, Ōya(tsu)hime, Tsumatsuhime, Ōtoshi, Ukanomitama, Ōyabiko, and Suseribime).

Legend states that a monk named Myōkō (妙行) had a vision of Gozu Tennō and the Hachiōji while meditating at the hill where the castle would later be built, Shiroyama (formerly also known as Fukazawayama), during the early 10th century.