Santa Claus appears and is referenced several times in Calvin and Hobbes.
He is usually mentioned when Calvin debates how good he should be at Christmastime. He does, however, appear entirely a few times: Calvin once imagined Santa Claus giving destructive presents to bad children and clothes to the well-behaved, leading to Santa's first full-body appearance in the strip (as well as a poem). Also, Calvin pictured Santa ranking Calvin's deservedness of presents based on evidence and accounts, and worrying about what Santa considered of him. Finally, in a weekday strip, as Calvin snowballs Susie, she mentions Santa, and Calvin pictures the face of an angered Saint Nick frowning down at him.