Tarzan is an alter ego of Calvin's who appeared twice in strips.
In his first appearance, Calvin held onto a swinging rope, and let go when he was close to the edge of a short cliff. When Calvin got back to land, Hobbes said, "The water looked a little cold, eh, Tarzan?"
In his final appearance, Calvin pounded his chest in front of Susie. Calvin was only wearing his underwear, and Susie asked if his mother knew he was doing this. Calvin then went home, grumbling that Jane would never say that to Tarzan.