Ikuchi

Ikuchi is a yōkai of the sea in Japanese legend. It is described in Tankai (譚海) by Sōan Tsumura and in Mimibukuro by Negishi Shizumori among other written works of the Edo Period.

According to "Tankai," in Hitachi Province (now Ibaraki Prefecture), the ikuchi is a strange fish that appears in the open sea, and it comes in contact with a boat once it discovers one, and even when it straddles the ship it is still able to pass, but since it has a body of several kilometers in length, it would require at least 12 koku (a little less than 3 hours) to pass it. From its body surface, an adhesive oil would seep out, and when it straddles the boat, it would spill a large amount of this oil on the ship, so if this is not scooped out, the boat would sink.

In "Mimibukuro," it was written as "Ikuji," and it was stated to appear from time to time from the western sea to the southern sea (Kansai region and Kyushu), and they would get caught on the bow of the ship. It is unusually long like an eel (ikuji), so that it would take 2 or 3 days for the boat to pass it, and it was stated that the proverb "ikuji naki" came from this. From the same book, a certain character said, "in the sea of Hachijō, Zushū (now Hachijō-jima, Tokyo, there are what one might think are small eel, but it is actually an eel-like creature that goes around in a loop, and since it moves without eyes or a mouth, so when it gets caught on the bow of a ship, it is not moving by extending itself, but rather revolving around."

In the entry "ayakashi" in Sekien Toriyama's Konjaku Hyakki Shūi, there is a drawing of a large sea snake, but it was drawn as ikuchi, an ayakashi (strange phenomenon of the sea).

During the present Heisei period, there have been hypotheses proposed that they are not strange fish, but rather giant sea snakes, or that they are humans who have drowned and want others to join them. Sekien's yōkai drawing resemble a sea serpent, a cryptid, some have identified them to be the same.

...
Wikipedia