 Two of the most prominent and standardized physical changes applied to rats through selective breeding are the development of the Manx and Dumbo . The Dumbo , whose origins are in the United States , is characterized by having large , low , round ears on the sides of its head , and was named for resemblance to the fictional Dumbo the Elephant . The Manx rat is tailless due to a genetic mutation , and was named for the Manx cat , which shares this feature , though not necessarily due to the same mutation . 
