Wererat

Wererats, also called ratmen, are humans who can transform themselves into three forms -- human, human-sized ratman, and giant rat -- due to their lycanthropy. They are sly and evil, and usually inhabit tunnel complexes beneath cities.

The wererat's human form tends to be a thin, wiry individual of shorter than average height. His eyes constantly dart around, and his nose and mouth may twitch if he is excited. Males often have thin, ragged moustaches. The ratman form is somewhat shorter than the human form. The head, torso, and tail are identical to those of a rat, but the limbs remain human. The third form is that of a giant rat 2 feet from nose to rump. This is the preferred form for travel and spying on potential victims.

Wererats are often followed by mundane mice or rats that are instinctively drawn to them but are not controlled by them.

Note: Wererats originally appeared in this form in the 2nd Edition Monstrous Manual for the AD&D game from TSR. Their use here is for the purposes of providing context for the campaign only.