Beast From the Cereal Aisle is a 15-page Sam & Max comic first published in 1992.
The Commissioner has Sam & Max investigate some "nasty business" at Slothmart Grocery World. After they get there and browse a bit the clerk directs them to the cereal aisle, where they soon come across increasingly bizarre paranormal activity. Eventually the monstrous Beast itself appears and starts to attack. Max manages to tame it somehow and turns it into a kids' ride. Sam and Max return home in a fire truck.