Ikuchi are colossal sea monsters that roam the open seas off the coasts of Japan. They appear in numerous stories from the Edo period, where they are described as enormous fish or monstrous serpents of some kind. Their bodies are covered in a slippery oil, which sheds as they swim the ocean.

When an ikuchi’s path crosses a boat’s, the sea monster envelopes the boat in its tentacle-like body. It slithers over the sides and across the deck, slowly sliding its whole body over the boat. Ikuchi are so long — many kilometers, by some accounts — that it can take hours for an entire one to slither over a boat. On a few occasions, boats have been tangled up in this monster for days. During this time, sailors must constantly bail the monster’s oily slime off of the deck to avoid being capsized by the heavy goo.

An ikuchi is depicted in Toriyama Sekien’s bestiary "Konjaku Hyakki Shūi", where it is called ayakashi. This yōkai is often referred to by that name. Ayakashi is more commonly used as a term for other strange creatures and supernatural phenomena and has nothing in particular to do with ikuchi. Toriyama Sekien may have just been listing the ikuchi as an example of an ayakashi. For whatever reason the name stuck.