Halonix Maximus

Halo is a mighty warrior from the cartoon portion of the Generation 1 continuity family... isn't he? He's also sometimes known as Halonix Maximus and Gigantron.

Let me see your giant Autobot hologram.

Decepticons scatter in fear at the merest sight of Halo. Towering over even the mighty Devastator, this incredible Autobot behemoth could surely shatter mountains and sunder the ground with his merest step. But he need not even take that step, need not set one foot upon the battlefield. The most meager of syllables uttered by his booming voice is enough to break the fighting spirit of the hardiest Decepticon and even put the fear of Primus into Megatron himself.

...to such an extent that they won't bother to check whether he's actually there, and not just a product of Hound's hologram powers.

"Nothing defeats the Devastator! Nothing!"
"NOTHING, LITTLE ONE? WHAT WOULD YOU SAY... ABOUT ME?"A hologram of Halo introduces himself to Devastator, "Heavy Metal War"

Fiction

The Transformers cartoon

Voice actor: Peter Cullen (English)

Believing that Megatron had legally defeated Optimus Prime in a one-on-one duel that forced them to leave the planet, the Autobots were compelled to ignore a battle raging outside their headquarters between the Dinobots and the Decepticons' newest warrior, the towering combiner Devastator. When Teletraan I revealed that Megatron had cheated in the battle, however, the Autobots immediately sprang into action. Lacking the physical power to take down Devastator, they turned to trickery and had Hound project a hologram of a massive Autobot, bigger even than Devastator, which mocked the Decepticon's claim that nothing could defeat him. The horrified Decepticons were stunned by the sudden appearance of this giant Autobot, and Optimus Prime exploited the opening to blast Devastator in just the right spot, forcing the Constructicons to disengage. The giant Autobot's purpose served, Hound shut off his hologram, and Megatron exploded with rage at the realization he had been tricked. Heavy Metal War

Timelines: Wings of Honor continuity

"Wings of Honor" stories take place in a continuity that includes all the events of the Generation 1 cartoon, but with some minor changes.

Halo was the leader of the Elite Guard's Artillery Team, also sometimes referred to as the 116th.

Presumably directed by Padlock, Halo rained death from above as three of Megatron's unfortunate warriors—Abacus, Datamine, and Deadbolt—bickered about the Decepticon Warlord's mission for them, accurately described by Datamine as "'His' only expectation is that we die!"

Back at Elite Guard Headquarters, Sentinel Major quipped about Big Bang's new "call sign"—"Gigantron"—for the giant Autobot, saying that "He loves it." "He", of course, referring to Big Bang rather than Halo. A Team Effort

Notes

• Given that he's just a hologram...where's his voice coming from? Considering his voice actor, however, it's incredibly likely it was meant for Prime himself to be voicing the giant Autobot.
• Despite appearing just the once, the giant Autobot was immortalized in super deformed glory as part of a large illustration of the entire cast of the cartoon, drawn by Hidetsugu Yoshioka for Pioneer's laser disc release of the cartoon in Japan. That's him on the extreme right, looking very much like a last-minute addition to the image.
• At BotCon 2010, it was revealed in Alex Milne's official lithograph and a TCC panel slide that the giant Autobot would be getting a name in an upcoming Wings of Honor story, and that he was a member of the Elite Guard's four-member Artillery Team.