Kitsune are foxes, not a separate species of spirit: in Japanese belief an ordinary fox is held to gain power with age, growing additional tails to a maximum of nine and acquiring the ability to take human shape, most often that of a woman. Two roles are usually distinguished. The zenko ("good foxes") are the servants and messengers of Inari, the deity of rice, agriculture and prosperity, and pairs of stone foxes flank the entrances to the many thousands of Inari shrines across Japan, often with a key or a jewel in the mouth. The yako or nogitsune ("field foxes") are tricksters or malevolent, given to deception, possession and seduction. The same animal thus stands at both ends of the moral scale, and which face it shows depends on the story and the shrine.
The most famous single kitsune narrative is that of Tamamo-no-Mae, a beautiful and brilliant court lady serving a Japanese emperor whose true form, revealed by a court diviner, was a nine-tailed fox spirit; hunted down after her exposure, she was killed and, by the fullest form of the legend, her spirit passed into the Sesshoseki, the "killing stone," said to poison or slay anyone who touched it until a Buddhist monk finally shattered and pacified it centuries later. Fox possession, kitsunetsuki, was also taken seriously enough as a folk affliction that it drew the attention of early Japanese psychiatric writers, among them Erwin Balz, who described it as a culture-specific condition in the late nineteenth century.