The idea that a fox grows in power as it ages traces back to an old Chinese book of marvels, the Genchūki^[2]. There it is explained that at fifty years a fox can transform into a woman, at a hundred into a beauty or a shrine maiden, and after a thousand years it gains passage to heaven. In Japan this notion that rank rises with age developed into a view that graded foxes as Tenko, Kūko, Kiko, and Yako.

It was the essays of Edo-period scholars that first set the four fox ranks down clearly. Minagawa Kien’s Yūhisai Sakki^[1] records the order, which was later introduced by Asakawa Zen’an’s Zen’an Zuihitsu^[3] and Tachibana Nankei’s Hokusō Sadan^[4]. Within this scheme the Kūko stands apart as a benevolent, high-ranking fox, distinct both from the harmful Yako and from the still spiritually shallow Kiko.

The Kūko, however, has left almost no concrete legends tied to a particular form or incident. This is because the word has been used less for one specific creature than as a marker of standing—an indication of just how exalted a given fox is. A fox that has cultivated virtue over a thousand years is sometimes called a senko, an immortal fox, but the Kūko and Tenko represent forms higher still. A fox standing at the very opposite pole from the people-deceiving Yako, dwelling almost in the realm of immortals and divinities: that is what the rank of Kūko signifies.