Rodimus Prime is a reformatted version of the Autobot Hot Rod.

Hot Rod

Hot Rod is a headstrong young Autobot that transforms into a futuristic sports car (best guess is something based off of a Lamborghini Countach). His love for action is both a blessing and a curse, as it makes him useful on the field, but often gets him into trouble. Hot Rod has a special relationship with Daniel Witwicky, serving as an oolder brother model of sorts. As well as his standard blaster, Hot Rod has been known to shoot plasma blasts from his arms, and also has used a retractable saw blade to fend off enemies. He is the Chosen One, and is responsible for the defeat of Unicron using the Autobot Matrix of Leadership.

Rodimus Prime

As Rodimus, Hot Rod is now reformatted into a taller, more imposing figure with different color distribution. His sportscar alternate form has now changed to reflect his new status as leader, much like Optimus Prime, and he can now transform into a "cabover Firebird" of sorts, keeping the same nose of his former car, but adding a large trailer to the back. Rodimus is stil Hot Rod at heart, and this means that he can still be somewhat hot-headed. He also bears the burden of partially contributing to Optimus Prime's death, and this combined with simple worrying that he will never live up to his former commander leads him to constantly second-guess himself. It should be noted that Rodimus is always Rodimus until or unless he passes off the Matrix to someone else.