Optimus Prime once just used to be one dude's name. But then Rodimus Prime happened and then the franchise decided that there were a bunch of _____ Primes that came before Optimus, and that "Prime" was a title. Thanks, Rodimus! I'm not sure how facetious that thanks is. Generally when the mythos is "grown" like that, it can be kinda irritatingly obtuse. But I think I'm fine with it. Usually.
It was decided in the Marvel comics, in the same brush stroke that decided that Prime was a title that we learned that the guy who came before Optimus Prime was called Sentinel Prime. It was just a series of name drops, but there was a panel from the UK stories a while prior that showed a yellow and orange and red robot handing the Matrix to Optimus as the former was dying. So that was probably Sentinel Prime. Years later, the Dreamwave comics would show us Sentinel Prime's forearm as he died, defeated by Megatron. It started to look like "we only get to see Sentinel Prime as he dies" was going to be a thing.
As Animated, the IDW comics, the War for Cybertron video game, and then later the third live-action movie gave us Sentinel Primes, it became obvious that Sentinel Prime, as he was forced to have an actual characterization, had to live in contrast with Optimus Prime. Optimus is the hero, and he kind of sucks up all the heroism in a room, and so Sentinel Prime became the jerk or the ineffectual leader or the secretly-kinda-genocidal-facist grandpa. Y'know, pick something that Optimus does really well, make Sentinel the opposite of that, and demonstrate why Optimus needed to show up and replace him.
The Titans Return toyline, much to my inner 12-year-old's boggling, now has a sweet G1-style Sentinel Prime toy. Despite having gone through so many iterations -- and despite obviously being Astrotrain painted to hopefully look like somebody else -- this toy still has a through-line to that first Marvel UK panel of the later-to-be-named Sentinel Prime. He's still orange and yellow and red, though the ratio of orange to yellow's been flipped, and he still has those three rectangles on his lower ribcage. That was one of the few bits of character design that managed to make it into the IDW reimagining of the original Sentinel Prime, those three rectangles. Artist Alex Milne made them into a cockpit window split into three, and here on this new toy they're no longer a cockpit window, but that detail remains all the same. The head (which now transforms into a tiny robot, Infinitus) harkens entirely to the IDW design, which covered up the mouth with an Optimus-esque mouthplate, albeit retractable. The rest is, well, an orange and red and yellow Astrotrain.
Despite essentially being a Voyager Class retry of the 2007 Deluxe Class Astrotrain, the larger toy feels less ambitious. Its transformation, at its core, is pretty simple. He's a robot, and then he pins his limbs together and he's a shuttle, and then he spreads out and he's an armored train. There's some small panel flipping here and there to cover up what you don't want to see (along with covering up the entire torso in train mode with a giant pile of shuttle wrapped around it like foil on a potato), but it feel perfunctory.
The packaging renders and early convention sample appearances led us to believe that his feet in robot mode are supposed to be train parts instead of the shuttle nose, but in practice this possibly not the best solution, as the instructions warn us to try the shuttle nose halves as feet instead. Otherwise, the angle of the feet makes it rough to stand the robot on on uneven surfaces like carpeting, but the train feet are generally fine on a very flat surface. I like trying to use those feet because they look "eviler." Just lots of sharp angles. The shuttle nose halves look like tap shoes, and as we all know tap shoes are benevolent.
As stated above, the head transforms into the tiny Infinitus. I feel like his name and future Astrotrain's head partner, Darkmoon, were switched at some point. Like, Sentinel Prime was in Dark of the Moon? Doesn't "Darkmoon" make more sense for him? And "Infinitus" could be a play on how Astrotrain is a space man, a la Buzz Lightyear with his infinity and beyond. Anyway, Infinitus pops off the shoulders, becomes a dude, and is meant to pilot Sentinel Prime in vehicle modes. He can do this either from the translucent orange cockpit on top of either vehicle or from the seat inside the wider of Sentinel Prime's two weapons. Other Titans Return toys have a smattering of tiny pegs to apply your tiny head dudes to, but Sentinel seems completely devoid of them, unless I've missed one tucked away. As such, he's missing a little play value. I like sticking those little head dudes on the bigger dudes' shoulders and whatnot.
Anyway, in the IDW comics, Sentinel Prime is gonna come back somehow and restore Cybertron to its former greatness probably. They never should have let him host SNL.