According to Japanese beliefs, Koromodako are strange and terrifying octopus-like yokai living in the seas bordering Kyoto and Fukui, particularly in the bays of Ine and Wakasa. Koromodako usually appear similar to ordinary small octopuses. Males only reach a size of a few centimeters long, while females can grow up to five times that length. Being so tiny, they are subject to the tides and waves, and so they float wherever the currents take them. Females live inside of a paper-thin shell, while males have no shell (similar to the family of octopuses called argonauts).

When koromodako are threatened they become incredibly dangerous. They can instantly grow to many times their original size — large enough to engulf fish, fishermen, or any other creature that might try to eat them. Stretching their arms and body out wide, they resemble an enormous piece of cloth, from which they get their name. While in this form a koromodako can engulf nearly anything in the water, even entire ships. It wraps its arms and mantle around the ship, sailors and all, and drags it down into the deep, never to be seen again. After feeding, the koromodako shrinks back down to its tiny size, impossible to trace.