Little Shop of Hamsters (Goosebumps Horrorland) by R. L. Stine
Sam Waters is desperate to have a pet. But his parents say that first, Sam must prove he's responsible. So he takes an after-school job in a pet store called Little Shop of Hamsters. To his horror, Sam soon finds that cute, little hamsters can become very UNCUTE monsters. Suddenly, Sam isn't fighting for a pet--he's fighting to survive! Could the souvenir he got in HorrorLand have anything to do with this?