The monster is a minor alter ego of Hobbes featured in among the Transmogrifier Gun alter egos of the fiirst Transmogrifier Gun story arc. Specifically, it originated in the Sunday strip in which Calvin fought with Hobbes over the Transmogrifier Gun.
The monster was the last of Hobbes' Transmogrifier Gun Alter Egos; Hobbes was returned to his normal form following the Sunday strip in which he first appeared.