From Halopedia, the Halo wiki
Jul 'Mdama's Covenant
These Unggoy are used to engage high-tier enemies through the use of augmented armor, explosives such as the Type-33 fuel rod gun and Type-58 fuel rod cannon. This class is composed of experienced and formidable Unggoy warriors, who are often fully capable of engaging light armed vehicles effectively. Unggoy of this role are rarely encountered on the battlefield, though.
Banished Unggoy Heavies were heavily augmented by machinery on Installation 00. Like their Covenant counterparts, they focus on heavy weapon use such as Type-33 fuel rod guns and Methane Wagons' plasma cannons. The Banished Unggoy Heavies are led by the Shadow Master of Buwan.
The original Unggoy Heavies wore distinctive sage-green armor, typically with a full-face methane breather and a cylindrical methane tank on their back, similar to the Unggoy Ultra. Their armor provided significantly more protection than that of an Unggoy Minor or Major, but it wasn't nearly as strong as an Ultra's or a Special Operations Unggoys in terms of durability.
Slim-stock tank harness
The Heavies of Jul 'Mdama's Covenant wear a helmet with full-face protective rebreathers, enhanced optical sensors with optics articulators, and ultra-dense battle harnesses that has a slim-stock tank with analog eye sockets of Unggoy predators. The harness is issued in orange, blue, silver, green, and magenta. The combat harness also allows for the Heavies to use the methane tank as a makeshift thruster pack, allowing for easy access in various terrains.
List of appearances