Calvin In Pirate Form is a minor recurring alter ego of Calvin's. He appeared in three weekday strips as a fantasy of Calvin's, roleplaying a pirate along with Hobbes.
1/9 1986Calvin In Pirate Form first appeared, along with Hobbes, wearing a newspaper hat and wielding a wooden sword in his treehouse. Much like G.R.O.S.S., Calvin's "fierce and dirty band of cutthroat pirates" was hostile to girls, excluding them from the imaginary ship. However, it predated the introduction of the club, and is as such unrelated. Hobbes, due to his noticeably romantic nature, opposed this exclusion.
6/21 1986Calvin In Pirate Form later appeared outside of his treehouse in a different outfit.
Although Hobbes had kept the paper hat and both had retained the wooden stand-in for a scimitar, Calvin had opted for a spotted bandanna as headwear. It is possible he was also wearing an eyepatch on the concealed side of his face.
The pirates had also left the treehouse for a more tangible representation of their ship: a wooden plank. Hobbes' enthusiasm for piracy seemed to have waned, given his sarcastic comment on being unable to walk the plank when "[the] ship [was] a plank".