According to Japanese folklore, a cat (neko) that has lived for a long time can become a kind of youkai called a nekomata (猫叉). It was believed that after a cat reached ten years of age, its tail would slowly split into two tails, and, along the way, it would develop magic powers, primarily those of necromancy and Shamanism. Nekomata are generally hostile to humans.
Example: Chen
Reference: Wikipedia entry