That is the House Mouse who, along with the Fruit Fly and E. Coli, may be the most important scientific lab animals that exist.
These cute, prolific little buggers have been carried around the world from their native origin in north India. They thrive in and around human habitat and, as the temperatures cool in the Fall, they are more likely to try to settle indoors.
Cats and mousetraps are made for them. It's October, and we have them now.