In both the G1 Cartoon and Marvel continuities, the Cybertronians developed the ability to transform in the early days of the war (in the show, the Autobots did it first; in the comics, the Decepticons did it first).
During the show, we see the Cybertronian Alt-Modes for some of the Cybertronians, but neither the show, or the comics (as far as I can find) ever mention or show what Megatron's Alt-Mode was prior to re-awakening on Earth and acquiring the new Walther P38 mode.
What was Megatron's original Cybertronian Alt-Mode in the G1 continuity?