A regular assumption in the fandom is that Optimus Prime and Ultra Magnus are lifelong pals, with a friendship or familial relationship that stretches back before the war. It results in a stream of fan theories, ranging from "Dion is Ultra Magnus!" to "The mystery Chaos Theory senator is Ultra Magnus!" always on the basis of "Ultra Magnus is Optimus's prewar buddy, he's the obvious choice." Is this actually based on anything? As far as I can tell it's based on Optimus saying "old friend" once in the movie, and while headcanoning two words into an extensive backstory then insisting that headcanon is actual canon certainly sounds like this fandom, there might also be some additional basis I'm not aware of. But honestly, I'm not even aware of any media... comic, cartoon, video game, anything... in which we see them hang out or even have a non-job-related conversation.