As man took to the stars in the distant past, he took many creatures from Earth with him, some intentionally, some less so. As in the age of sail, wherever Man went, Rat went as well.
Over tens of thousands of years some changed. Exactly what the catalyst was is unknown - perhaps the influence of the warp, perhaps the continued effect of experimentation with the sciences of the Dark Age of Technology, perhaps an intentional uplifting to create disposable workers or soldiers, perhaps a combination of them all, mixing and breeding. But regardless of the cause in the 41st Millenium another race contests the stars, often from within and underneath the works of the Imperium.
While some consider them a form of the aberrant beastmen, offshoots of humanity itself, or some strand of alien, this is not the case. Known by various names - Ratmen, the Nimh, and many others this plague lurks ready to strike when conditions are right.