A monster best known as an image appearing in the Gazu Hyakki Yakō and various picture scrolls, where it takes the form of a little old man with a huge, elongate cranium. Sekien's drawing shows this fellow slipping out of a palanquin and through an open doorway. No doubt he is up to no good, as this creature's name is derived from vernacular terms implying a slippery and elusive quality, akin to the proverbial catfish and gourd.

No description was left with any of these images, but in modern popular culture nurarihyon has acquired the impressive status of the supreme commander of yōkai, and the peculiar habit of sneaking into human houses in the evening while everyone is busy, making himself at home and drinking the tea. A variety of twentieth century authors seem to have contributed to the evolution of this persona, starting in 1929 with a short picture caption in Fujisawa Morihiko's 藤沢衛彦 Yōkai Gadan Zenshū 妖怪画談全集:

まだ宵の口の灯影にぬらりひょんと訪問する怪物の親玉
In the flickering light when it is not yet nightfall, The boss of monsters visits with a nurarihyon.

In 1976, Satō Arifumi 佐藤有文 described the nurarihyon in his Nihon Yōkai Zukan 日本妖怪図鑑:

年の暮れになると、どこからともなくあらわれる妖怪。とてもいそがしいのに、
家の中にはいりこんでどっかりすわる」と創作する。
A monster who, at the end of the year, suddenly appears out of nowhere. Although things are very busy, he comes into the house and plunks himself down.

Such ideas were built on, exaggerated and elaborated in Japan's many popular publications on the subject of yōkai, until today's nurarihyon was fully developed, but ultimately this nurarihyon is imaginative fiction based on Sekien's image and not a historic concept.

Not so celebrated or powerful is the nurarihyon of folklore, a weird sea monster found in the Bisan Strait of the Seto Inland Sea off the shore of Okayama Prefecture. A bulbous, floating mass vaguely resembling a human head, it bobs in the waves until someone in a boat attempts to take it, at which point it sinks to the bottom (nurari), only to pop up again (hyon) a short while later. It will repeat this behavior again and again, seeming to tease whoever takes an interest in it. Tada Katsumi has noted that this nurarihyon may be a real animal (a Portuguese man o' war or some other large cnidarian) regarded as an uncanny being.


Citations
Murakami 2005 p. 247, Inada p. 84, Tada 2000 p. 149-50. Sekien apparently called this monster nūrihyon, but this might be a mis-written character - hiragana ra ら and u う are similar.