These are humans who can turn themselves into rats at will. No one quite knows how they came to do this, as it doesn't seem to be contagious.
Advantages: Alternate Form (Rat) ; Night Vision 8 ; Reduced Consumption 2 (Cast-Iron Stomach, -50%) ; Resistant to Metabolic Hazards (+8) .
Features: Use the Rat template on p. 24 of GURPS Dungeon Fantasy 5: Allies, but note the cost of Alternate Form is reduced as he has some of those traits in human form, as well as skills.
In human form, they will often have rat-like features like a small nose, a pointed chin and salt-and-pepper hair.