A fancy mouse is a domesticated form of the house mouse, one of many species of mice, usually kept as a type of pocket pet. Fancy mice have also been specially bred for exhibiting, with shows being held internationally. A pet mouse is inexpensive compared to larger pets, and even many other pet rodents, but mice are comparatively short-lived: typically only 2 to 3 years.

The house mouse is a small mammal of the order Rodentia, characteristically having a pointed snout, large rounded ears, and a long and almost hairless tail. It is one of the most abundant species of the genus Mus. Although a wild animal, the house mouse has benefited significantly from associating with human habitation to the point that truly wild populations are significantly less common than the semi-tame populations near human activity.