Gremlins

Gremlins is a movie starring Zach Galligan, Phoebe Cates, and Hoyt Axton. A young man inadvertently breaks three important rules concerning his new pet and unleashes a horde of malevolently mischievous monsters on a small town.
When Billy Peltzer is given a strange but adorable pet named Gizmo for Christmas, he inadvertently breaks the three important rules of caring for a Mogwai, unleashing a horde of mischievous gremlins on a small town.