Primal Rats

Primal Rats are giants that transformed into Rats and Rat-Hulks are the progeny of these rats. They appeared during the first war when food became scarce and giants looked to the sewer rats (normal, non-humanoid) and ate them in an attempt to stave off starvation until they had won the war.

The corruption spread from the rats to the Giants and soon enough they would transform into the first kind of humanoid rat.

Smaller humanoid rats were normally sent to die as the larger ones would trample friend and foe on their war path.