Wererat

A humanoid that can (or must) change into either a rat or a half rat, half man sort of man-rat monster man. They are immune to weapons that are neither silver nor magical. Therefor, a weapon must be either magical or silver to harm them. They can regenerate.

There are two types of wererats, turned wererats and true wererats. A true wererat was born a wererat, while a turned wererat has been bitten by a true wererat and is thusly cursed. A true wererat can change into any of it's three forms at will, while a turned wererat must change on the fullmoon into it's man-rat monster man form. When under the moon's influence, the turned rat looses control of it's faculties, and is known to rampage.

It is unclear at this point how much control a turned wererat has over it's abilities when the moon isn't full.

So far, the only true wererat encountered has been Norwit. If you're looking to find a wererat, just go to Calpe and throw a rock.