white rat; albino rat
Refers to the domesticated albino rat, often used as a pet or in laboratory settings. The term is not limited to the mythological context.
I bought a white rat at the pet shop.
Albino rats are often used in laboratories.
Daikoku's white rat; white rat of Daikokuten
In Japanese folklore, the white rat is considered a messenger or servant of Daikokuten (大黒天), the god of wealth and prosperity. This sense is specific to the mythological context.
The white rat, believed to be a messenger of Daikokuten, is said to bring good fortune.