I’ve gone into some of the origin of Megatron’s original toy before, but the brief version is that the toy that would become Megatron was originally the twelfth figure in Takara’s Micro Change toy line, named “MC-12 GunRobo.”
Since Hasbro has never been able to re-release the original Megatron toy, due to the changing of US toy safety laws, it remains one of the only 1984 Transformers toy that never got recolored into a different character. So the only thing I could do when repainting my Robot Heroes Megatron was go back to his origins, and paint him up as GunRobo:
Transformers, Megatron, Micro Change, and GunRobo are all owned by some combination of Hasbro and Takara.