A Tesso shapeshifts into a rat. It's unknown if his true form is the rat.
It is also a powerful shaman and can absorb knowledge by eating it.
Tesso (The iron rat) is a one of a kind yokai. A rat demon the size of a full-grown man, armed with metal teeth and claws, capable of shredding and tearing almost anything. Combine this with a pied-piper like ability to call a legion of real rats to do his destructive bidding, and Tesso becomes quite a formidable foe to be reckoned with.