Zombie Calvin is one of Calvin's imaginary Forms. Calvin has "risen from the dead" several times in Watterson's strips (usually as an excuse to skip homework)
Calvin and Hobbes in their Zombie alter-egos.
. Hobbes has also joined his friend in an "undead walk" to which Hobbes and Calvin have stared at one another and eventually begin laughing violently.
"Of coarse, real zombies don't get the giggles while looking at one another!"---Calvin