The octopus in the sleeve
Off the northern edge of Kyoto Prefecture, where the Tango Peninsula juts into Wakasa Bay, fishermen knew to look twice at anything small. A tiny octopus in the water is nothing. A tiny octopus is a dress folded up small. They called it the koromodako, the clothing octopus, and its whole trick is scale: it waits as something you could cup in one hand, and when a boat comes close it spreads out like a garment shaken open — six tatami mats wide, by one telling — and takes the boat, the nets, and the men down with it.
Sit with what the name does. The yōkai catalogs are full of things that were never small: serpents, spiders the size of horses, the sea itself with a face. The koromodako is rarer. It is a monster that begins at the size of a garment a woman has taken off, something intimate enough to hold, and its danger is exactly that intimacy. The boat that sinks is the boat that came closest. Nothing here leaps out of the dark. It waits to be picked up.
The water under the story is real. The Sea of Japan keeps the blanket octopus, a creature whose memory-laden membranes spread ten times its body width when it wants to look large, and on parts of that same coast fishermen call the real animal by the same name, koromo-dako. So the layers read clean: a real animal that inflates like cloth, a warning story wrapped around it, and the warning is old enough that Tokihiko Ōtō, writing in 1949, could pinpoint the village of Sodeshi at the tip of the peninsula as the place where the tale was kept.
One more turn, since it is late and you came for the sharp part. Every telling agrees the koromodako looks like something you could fold away and keep. It never looks like a monster first. The night you find out what it is, you are already holding it.
Sleep with your sleeves folded flat. And if something small drifts near the boat, admire it from very far away.
Sources
Koromodako (ころもだこ / 衣蠨, “clothing octopus”), a yōkai of northern Kyoto Prefecture: appears as a small octopus, expands like a spread garment (one telling: six tatami mats, about 13.2 m²) to envelop and sink boats and people
Localization: promontory areas of Wakasa Bay; Sodeshi village at the northern tip of the Tango (Yosa) Peninsula per Tokihiko Ōtō (1949)
Real-animal comparison: the blanket octopus (紫銓, murasaki-dako), whose spread membranes expand its apparent area about ten-fold; the same name koromo-dako is used locally for the actual animal in parts of Japan (e.g. Takeno, Hyōgo)