According to japanese folklore, Kani oni are demonic crabs with blue-green, spiky carapaces. The faces are whiskered, with blue lips, and eyes which roll in opposite directions. Their wide mouths are full of sharp, black teeth, and tusks protruding from the corners of their smile. Unlike normal crabs, it only has six limbs.

While their odd appearance makes quite an impression, nothing is known about what kani oni do or how they live. Everything about them beyond their appearance is a mystery.

Kani oni comes from Bakemono zukushi emaki, and like the other yōkai found in this anonymous Edo period scroll, no description of them is given beyond an illustration. While many of the yōkai painted in this scroll are based on puns names which allow us to speculate on their nature, kani oni simply means “crab demon” and does not leave much room for interpretation. The blackened teeth suggest the old fashion of ohaguro, which often gave its users a frightful appearance. The spikes on its shell, its tusks, its blue-green color, and its facial hair all give the impression of a classical oni.

There is a species of crab known in Japanese as kimengani (Dorippe sinica) or “demon-mask crabs”, so named for the markings on their carapace which resemble the face of a scowling oni. They are related to another species of crab with folkloric connections, heikeigani (Heikeopsis japonica), which are named for the markings on their carapace which resemble samurai warriors. It is possible that kani oni may have been inspired by one of these crab species.