The Norway or brown rat, as it is sometimes called, is not always brown, did not originate in Norway and was classified first as Mus and later as Epymus before the Linnean classification as Rattus norvegicus by Berkenhout in 1769. The appellation ‘Norway rat’ derives from Berkenhout’s assumption that the first R. norvegicus arrived in the UK on lumber ships from Norway, although R. norvegicus was probably not present in Norway in mid-18th century, when the species first invaded Britain. Wild R. norvegicus, the forebears of all laboratory rat strains whether albino, hooded, black or agouti, are thought to have originated somewhere in Asia, possibly in the plains of northern China. So, perhaps, we should call them Chinese rats.
("Norway rats", by Bennett G. Galef. Current Biology Volume 19, Issue 19, 13 October 2009, Pages R884-R885)