At the end of the first century, the infant Fox Clan managed to survive in its strange new home by forming an alliance with the fox spirits of the Kitsune Mori - an arrangement that ultimately included intermarriage with the spirits in their human form. From time to time this spiritual heritage manifests itself in a young Kitsune who seems to communicate with animals instinctively who is more at home in the forest than in a house or temple, and who begins to show strange physical affinities to the fox spirits: green eyes, red or white hair, and even fur.
In the early days of the clan, these strangely gifted individuals often lost their humanity to the allure of the fox spirits' wild and unfettered existence. Over the centuries, however, the Kitsune shugenja learned to recognize the symptoms of those whose connection to Chikushudo was dangerously strong. Eventually the family devised techniques of self-control and spiritual harmony that would let these individuals find a balance between their human and animal sides.
-- "The Children of the Forest" Sidebar, page 65, The Book of Earth
So it seems the passage in question was not a Disadvantage in a mechanical sense but simply described what seems to be one. I am now reasonably convinced that the best ways to represent this are by reskinning certain other disadvantages (eg: Disturbing Countenance, Lord Moon's Curse) or better by using the Child of Chikushudo Advantage found on page 244 of Enemies of the Empire, which directly reflects a spiritual heritage.