Having been reinvented numerous times and having been cannibalized in all sorts of toy lines, it's not always known that Transformers was also initially a toy series that came from many, many others.
The toy line came from several sources, the two most well-known were Takara's own Microman and Diaclone toy lines. Several toys, including Optimus Prime, were originally released under other banners and other names, like Convoy. Hasbro saw the marketing potential in transforming vehicles, brought them to the US under a new brand name, and the rest is history. Aside from these Takara toys, the Rhode Island toymaker also snagged some BanDai items, the two most famous of which were Omega Supreme and Jetfire.
Since the original generation, it's been reinvented as Generation 2, Beast Wars, Machine Wars, Beast Machines, Robots in Disguise, Armada, Alternators, Universe, Energon, Cybertron, Classics, Animated, two movies, and now Generations plus Power Core Combiners.
There are dozens of excellent sources for additional information on these robots in disguise on our link page.