In fairness, there are other North American animals that have crossed over - I got curious enough to look it up just now and found turkeys, guinea pigs, and some kinds of North American muskrats and squirrels have established themselves in Europe. And apparently there are feral raccoons in Germany that came from imported animals that escaped. So I wouldn't be too hard on fanfic writers who haven't made a point of studying up on specific rodent populations, y'know?