Ōnyūdō

Ōnyūdō is not given a single canonical origin across regional traditions. Local accounts variously explain Ōnyūdō as a giant monk-like apparition, as an optical or atmospheric phenomenon (for example, steam or a Brocken-spectre-like halo at hot springs), or as the manifestation of an animating animal or object: foxes (kitsune), tanuki, weasels, otters, or animated stone pagodas. Some tales treat particular manifestations as warnings from mountain deities or as connected to human deaths (wartime soldier apparitions), but the sources emphasize regional variability rather than a unified cosmological origin.

Descriptions vary widely: many accounts depict a towering, monk-like figure (hence the name meaning "big monk"), while others report a vague giant or shadowy humanoid. Regional reports give a broad range of sizes—from around two meters in some tales to figures described as several jō high or even "as large as mountains" in other local accounts. Specific sensory details in some narratives include very large staring eyes that reportedly cause fainting, and in a hot-spring report a seven-colored aureola surrounding a giant shadow.

Folkloric behaviors attributed to Ōnyūdō differ by story. Some legends claim that observers who see an Ōnyūdō may fall ill or faint, particularly when stared at by its large eyes. Other accounts describe menacing conduct—threatening or attacking travelers, or menacing people who watch it perform tasks (for example, a tale where it hulled rice while menacing any onlooker). Many regional narratives ultimately explain an Ōnyūdō as an illusion or transformation: the apparition vanished when an animating animal was killed or when shot (a Date Masamune account where shooting revealed an otter), and some sightings are plausibly attributable to atmospheric optics.