water vole 
noun
A large semiaquatic vole which excavates burrows in the banks of rivers.
- Genera Arvicola and Microtus, family Muridae: three species, in particular the European water vole (A. terrestris) and the American water vole (M. richardsoni).
- A huge decline in the number of Britain's native mammals - such as the water vole and dormouse - has led to the launch of a new organisation which is seeking the support of animal-lovers to help to save our furry friends.
- Rats have quite large ears unlike voles, so Ratty is in fact a water vole not a rat.