They could be Cybertronian exiles who became space pirates being pursued by the Autobot Elite Guard (or something), who deliberately take the form of robotic dinosaurs just to be mischievous when they arrive on Earth. They're not inherently bad, and eventually side with the Autobots to fight whoever the main villain is. At the end of the movie they escape to get away from the Elite Guard who plan to arrest them after the primary villain is neutralized. Grimlock, of course, is the captain. I imagine one of the humans being scared shitless (like Sam was by Ironhide) only much worse by Grimlock. Then Grimlock proceeds to tease him with his fiery sword until -get this- Ironhide tells him to cut it out.