Calvin appeared as an owl alter ego in among the Transmogrifier Gun alter egos of the first Transmogrifier Gun story arc (2/8/88 through 2/20/88). Specifically, it originated in the Sunday strip in which Calvin fought with Hobbes over the Transmogrifier Gun.
Calvin intended for Hobbes and him to return to their normal forms, but the Transmogrifier Gun malfunctioned after Hobbes was restored. Stuck in owl form, he was dismayed by his condition until he realized that it would keep him from attending school. Calvin remained as this alter ego for under a day, changing back into a boy overnight as a side effect of the transmogrification. As such, this was the last of Calvin's original Transmogrifier Gun alter egos.