The word rat, however, it a species or even a group of species, but is roughly defined as "things that look like a rat" making it awfully self-referential in that one must know what a "rat" looks like. And they can look way different and be in entire different families. A similar situation occurs with "bats" which also cover a lot of different families but in general, if you see one, you know it is a bat.
A muskrat (Ondatra zibethicus) sorta fits this mold of looking like a rat (and hence the name) but for distinction added is the prefix "musk" because they have a gland that releases an odor.
All that said, here is a picture of one eating dinner at a pond at the Meadowlark Botanical Gardens close to Vienna, Virginia.