Mizuchi.

Mizuchi (蛟, みずち) is not a neatly fixed dragon with one standard body. In the oldest texts it is a dangerous water being, imagined as a great serpent or dragon that inhabits deep river pools and poisoned crossings. Later readers often picture it with a long scaled body, hornless dragon head, and river-colored hide, but the classical materials care more about where it lives and what it does than about a full anatomical portrait.

That uncertainty matters. Mizuchi belongs to an early layer of Japanese water lore in which rivers are powerful presences with wills of their own. It is less a decorative dragon than the spirit of a place where water turns deadly, where a ford becomes a grave-site, and where the current seems to test human courage and ritual intelligence (Nihon Shoki, Book 11).

Mizuchi enters legend as a killer of travelers. In the Nihon Shoki account from Kibi, passersby who approach its waters are poisoned and die, until Agatamori confronts the creature. He throws floating calabashes onto the river and demands that the beast sink them if it truly rules the place. When the mizuchi fails, he slays it and then destroys the other water-dragons hidden below the pool.

The pattern is important across later interpretation. Dangerous water is answered not only with weapons but with ordeal, taboo, and proof. Folklorists connect that calabash challenge to later kappa traditions in which river beings also fear or fail before gourds, suggesting that mizuchi survives less as a single famous monster than as an older layer beneath later Japanese water yōkai.