The X-387 Robot Probe is a minor alter ego of Calvin's. He appeared in a single weekday strip, as a probe sent from Jupiter to sample Earth materials. To do so, it visited Earthlings and demanded to obtain certain substances at gunpoint. Calvin had built the probe in order to obtain some chocolate.
The probe was an upside-down cardboard box with the flaps out and primitive facial features on the front. Underneath these, there was a small hole out of which the probe could extend an "analysis scoop". The hole also allowed an annihilator weapon (which was actually Calvin's dart gun) to be brought forth and fired at non-compliant people.