Imperial Guard commanders may when expedient press abhuman races into service when conscripts or other manpower resources dry up. Inquisitorial or Ecclesiarchical certificates of exemption and approval are usually sought by cautious commanders just in case they fall foul of local planetary regulations and find their newly formed units exterminated for the heresy of mutation.
The Ratkin are not actually mutants but are deviant humans who have sought escape from the stresses of Imperial Civil life through the cult of the Rattus Immortalus Perfectus. This cult is supressed by the Inquisition but as it reappears regularly hopping from world to world stowed away on starships many take the pragmatic view that these toughened and aggressive creatures that can be trained to obey orders might as well serve the emperor as cannon fodder as in their death they will be returning to the imperium some measure of what they owe for their life and upbringing.
