Decepto-Pack
The Decepto-Pack is a backpack designed to be worn by Humans. Upon activation it transforms into a full set of Decepticon combat armor for the wearer.
The armor consists of a breast plate, belt, wristbands, Transformer helmet with faceplate, an arm shield, and pistol-style laser weapon. All pieces of the armor are visibly marked with the Decepticon symbol. The shield also bears the likeness of an unknown Decepticon warrior. (Whether this face represents some autonomous Decepticon intelligence existing within the Decepto-Pack is unknown.)
It is assumed that a human or group of humans outfitted this way would be able to aid the Decepticons in combat with the Autobots. It is possible that the Decepto-Packs were created to counter the Autobots' aggressive S.T.A.R.S. program. It is unknown if the bearers of the packs would have been mind-controlled conscripts or voluntary converts.
Fiction
Ask Vector Prime
With the success of the Autobots' S.T.A.R.S. program, the Decepticons of Primax 185.0 Beta responded with their own Decepto-Packs, enslaving humans and giving them the Decepto-Packs to augment their capabilities and help him conquer the planet. Galvatron eventually provoked a nuclear strike that provided him the energon necessary to build Zod drones which he unleashed across the reality, conquering many systems, though the Quintessons finally interfered and both forces went to war. Galvatron was finally stopped when his spark was destroyed as he was transplanting it into Cybertron itself to become Grand Galvatron. The war continued for several more centuries, the Galactic Council, representing the broken universe, blaming humanity for the war. Ask Vector Prime
Toys
Generation 1
The Decepto-Pack was sold by HG Toys in 1985. It's the only actual transforming Transformers "costume" marketed to the public. Why they chose the Decepticons over the Autobots is unknown, except perhaps that bad guys are just cooler.
Packaging variant: The Decepto-Pack was sold featuring either a cheerful blond boy in a yellow shirt, or a sullen brown-haired boy in an orange shirt. The boxart differed completely between variants. Presumably the brown-haired version came later, since it matches the live model.
