A brown rat.
brown rat definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
a common, omnivorous large rat (Rattus norvegicus) that swims and dives well, usually lives in underground burrows, and spreads various diseases: the mutant form, a white rat, is used for medical experiments
brown rat definition by American Heritage Dictionary
See Norway rat.