Kumiho

Its true form is a fox with nine tails

Its true form is a fox with nine tails, and in some tellings the number and condition of its tails signal its age and power.

The Kumiho can typically be exposed by its shadow, its reflection, or the fact that it retains a hidden fox tail or fox-shaped ears even in human disguise; in several well-known tales, a suspicious husband or monk uncovers the creature's true nature through one of these tells and it is driven off or destroyed.

Its most notorious trait is the hunger for human livers or hearts, which in many stories it must consume, often one per night over a set number of nights (frequently a thousand), to complete its transformation into a full human being or to sustain its power.

it retains a hidden fox tail or fox-shaped ears even in human disguise

The Kumiho's signature power is shapeshifting, especially into human form, which it uses to infiltrate villages, marry unsuspecting men, or seduce travelers.

Some tellings hold that it carries a fox bead (yeowoo guseul, 여우구슬) in its mouth that is the source of its magic, and that a human who steals or swallows the bead can gain knowledge or power, though doing so is dangerous.