Fancy mice are domesticated breeds of the common or house mouse

House mouse

The house mouse is a small rodent, a mouse, one of the most numerous species of the genus Mus.As a wild animal the house mouse mainly lives associated with humans, causing damage to crops and stored food....

 (Mus musculus). The terms fancy and "feeder mice" are often used interchangeably by retailers, and are in fact the same variety of mouse.
Fancy mice is a term to describe mice that have been selectively bred for exhibition. They can vary greatly in size, from small pet mice that are approximately 15 centimetre long from nose to the tip of the tail, to show mice that measure 30 cm (11.8 in) nose to tail.