Gilthor
Guiltar was a Quintesson creation from the Generation 1 series.
Guiltar, also known as the "Great-Gun Soldier", is a bizarre, vicious fusion of Optimus Prime and Megatron, sent to attack both Autobot and Decepticon alike. It is ultimately a lifeless shell created by the Quintessons, animated by the "ghost" spark of Starscream.
It has a massive artillery truck mode, as well as multiple gun configurations for its robot mode. Physically, it can hold its own against many of the toughest Transformers.
(Note: The Romanization of the character's name given here is not "official"; there isn't one. There are any number of ways to spell it from the katakana given. This is just the simplest one that seems "right".)
Fiction
2010 Manga
Guiltar rose from the sea off of Japan, seeking out Rodimus Prime, who was engaged in battle with Galvatron. Guiltar attacked Rodimus with a variety of artillery modes, sending the Autobot leader flying.
Gavlatron revelaed that Guiltar was a Transformer created by the Quintessons, who had risen up the ranks of the Decepticons... because apparently, Galvatron doesn't have any accumulated memory whatsoever. As Galvatron gloated over his fallen enemy, Guiltar snuck behind him and attacked, savagely beating Galvatron and kicking him to where Rodimus had fallen. Guiltar was gunning for leadership of the Decepticons, which even a concussed box turtle could have seen coming. But not Galvatron.
Down, the two faction leaders teamed up against their new common foe. Rodimus transformed into vehicle mode and rammed Guiltar, Galvatron following up with a blast in cannon mode. The two joined in a combined kick, destroying Guiltar's body. Starscream's ghost floated free of the shattered chassis, and Rodimus reminded Galavtron that, obviously, the Quintessons could not be trusted. The two went their separate ways for now, both too weak from Guiltar's attacks to continue the fight.
