Calvin the Zombie is a minor alter ego of Calvin's. It appeared in a single story arc (4/27 through 4/30 1987). In it, Calvin was pretending to be a reanimated corpse in order to avoid having to do his homework: he believed that the living dead do not need to solve problems. Hobbes went on to imitate Calvin upon seeing his expression.
Calvin also later appeared in a Sunday strip in which he undergoes various transformations. One of these is a zombie.