It's seemed a very odd choice since Legends Series Ultra Magnus was revealed that the Minimus Ambus figure that rides inside Magnus would be decoed to resemble Alpha Trion. In reality it's probably mostly to do with having a vaguely similar general design, but what we've been curious about has been the story explanation. And thanks to the latest Legends web comic pages, we finally have an answer! Keep reading to learn more, and see Megatron in a suit!
One of the two new comic pages reveals the backstory of Legends Ultra Magnus. As seen in the comic, Battle Convoy (G2 Laser Optimus) is having some difficulty with Galvatron II, and so communes with the Matrix, contacting Alpha Trion within to go find Ultra Magnus who had been killed by Sixshot. The spirit of Alpha Trion joins with Magnus's spirit, which allows Ultra Magnus to be restored to his body. ...which had been buried at sea. Magnus fights his way out of his coffin and resurfaces, just in time to catch the Headmasters by surprise, who flee in terror from the reanimated form of Ultra Magnus, now reconfigured to resemble the new toy. Finally, Alpha Trion spills out of Magnus's body, having apparently hitch hiked his way back to life as well!
So, in the grand tradition of things like Matrix Shine Ultra Magnus, the Legends toy seems to be designed to represent a single moment of time, in this case Magnus's immediate post-revival with Alpha Trion still hidden inside. This also puts this form as an interim stage, since the Japanese G2 setting implies Ultra Magnus will be taking up his Laser Rod body soon after this establishing story. Meanwhile in the second comic page, Megatron is wearing a suit, and I don't think anyone needs more context than that, right?