Jyuunishi Tsuki
Possessed by the 12shi

Certain members of the Sohma clan are possessed by the different spirits of the jyuunishi (creatures from the Chinese horoscope) and the cat. Under certain circumstances, those who are possessed, transform into their animal forms. After some time, they then return to human form, however, the amount of time it takes to revert to human form is variable.

This curse has continued for several hundred years and the specific circumstances that determine who will be possessed is not clearly understood. Members of the same family are not necessarily more likely to be possessed. Either which way, having family members like Yuki and Ayame being possessed is quite unusual.