This hunched creature looks like a human in studded leather, but fur covers its body. Its face is rat-like, and it has a long, naked tail.
5e SRD
A thin, wiry lycanthrope that can assume the form of a giant rat or ratman
A wererat in human form tends to be a thin, wiry individual of shorter than average height, with eyes constantly darting around. A wererat's nose and mouth may twitch if he or she is excited. Males often wear thin, ragged mustaches.
In addition to assuming the form of a giant rat, wererats can assume an intermediate form (a "ratman"). The ratman form shares the animal form's immunity to normal weapons, and can deliver an identical bite, but in this form the wererat may use a normal weapon instead of biting. Note that the wererat in ratman form cannot bite and use a weapon in the same round.
Unlike most lycanthropes, wererats prefer to inhabit civilized areas, particularly cities. They frequently lair in sewers or other underground areas, coming out by night to steal from or kill city folk.
Basic Fantasy Role-Playing Game
Shapechangers with a human and an animal form.
OSE
Intelligent, humanoid rats who can change into normal humans
The horrid ratmen infest the underground sewers and catacombs under large cities. They are stealthy and surprise on a 1-4. They can assume any of three forms: human, giant rat, or man-rat. In human or man-rat form, they often arm themselves with swords, but prefer to lure unsuspecting individuals into traps where the victim may be held for ransom or later meals. A wererat may summon 2d6 giant rats and control them. Rumours persist of a city of black wererats on a remote shore; the more fanciful tales suggest the black wererats may have powerful spell casters among their number who can magically affect the phases of the moon.
OSRIC