The Driving Robot is an enemy cut from Serious Sam: The First Encounter.
The Driving Robot was a robot that would drive around on the ground. It came in two variants, R2-D2 and Spider. It had two attacks; firing a laser and launching a bomb. The code for these weapons come from the Cyborg and Beheaded Bomber, respectively. The Driving Robot's low health and score seems to suggest that it was a cannon fodder enemy, like the Beheaded Rocketeer.
Based on its looks, the Driving Robot would at least be in Sky City, as both locations have a mechanical and futuristic look.
The Driving Robot apparently never got it's own sounds before being cut, as all of its sounds are from the Minor Biomechanoid.
The Driving Robot was removed from the final game. The only remnants of it are some code files in the final game and in the Serious Sam 1 SDK.