竜宮（りゅうぐう）
Ryūgū
The Dragon Palace under the sea, and the beings that come from it.
Ryūgū is the palace of the dragon king at the bottom of the sea, where time runs slowly and the fisherman Urashima Tarō spent three years that were three hundred on land. It is the other world that the sea opens onto, as the mountain opens onto the world of tengu and the deep forest onto the world of the gods.
Its people appear on the shore. The samebito of Lafcadio Hearn’s telling is an exile from the Dragon Palace whose tears turn to rubies; the ningyo, the fish-bodied person, is cut up and eaten and gives a girl eight hundred years of life; the umi-bōzu rises from the same water with no palace behind it at all. Sailors who met something strange and could not name it said ayakashi.
Related yokai
Samebito
鮫人（さめびと）
In a story Lafcadio Hearn published in English in 1900, a man on the Long Bridge of Seta shelters a creature banished from the Dragon Palace — and is repaid in tears that harden into jewels, which only come when the grief behind them is real.
Ningyo
人魚（にんぎょ）
The Japanese ningyo of the old illustrated encyclopedias is not a mermaid — it is a fish body carrying a human face, with no arms and nothing to seduce anyone with. The famous story attached to it is not about romance either.
Umi-bōzu
海坊主（うみぼうず）
One account describes a vast Buddha-like shape crossing ahead of a boat on a clear, windless day, then slowly thinning until it vanished.
Isonade
磯撫で（いそなで）
In the 1841 Ehon Hyaku Monogatari a shark-like creature of the western sea attacks with its tail — and in Kumano in 1960 the name was still the thing people said when someone died at the shore.