That means that the terms rat and mouse are not specific in a taxonomic way : If a large muroid is discovered , it will often have rat in its name , if it is small , it will often be called mouse - this does not mean , however that the animals discovered will be part of the genus rattus ( for rats ) or mus for mice . Generally , when someone discovers a large muroid , its common name includes the term rat , while if it is small , the name includes the term mouse . Scientifically , the terms are not confined to members of the Rattus and Mus genera , for example , the pack rat and cotton mouse .