Honrai no Sugata
Ultimate Form

Even among those who are cursed, the cat is the exception. In reality, the cat has one other form. That form is so extreme, it's essentially that of a monster. This is the ultimate form of those possessed by the cat and is normally kept sealed away by the juzu beads worn on the left hand. Because of this, those possessed by the cat is abhorred among the Sohma's. The one possessed by the cat before Kyo, Kazuma's grandfather, was essentially locked away and his life ended in this manner.

Why did the spirit of the cat which is not one of the 12shi come to haunt the Sohma clan. Why does only the cat have that extreme form. Those reasons have yet to be revealed. According to Kyo, the spirit of the mouse who possesses Yuki is the one that is at fault for that but......