From Halopedia, the Halo wiki
So-called "flak" hunters are a rank or role of Mgalekgolo gestalt. They are primarily distinguished from their typically-deployed brethren thanks to their distinctive gold armor. Additionally, they wield a variant of assault cannon that fires single shots of incendiary gel, while their blue-armored comrades fire streams of the gel. The "flak" hunters' role is unclear within the Covenant, as other hunter types have been observed with the same (or similar) weapon systems as the "flak" hunters. These gold hunter variants are deployed in both exclusive "flak" pairs, and as paired with a regular "blue" hunter.
"Flak" hunters were deployed during the Battle of Mombasa in October 2552. Following the departure of the Prophet of Regret, the Jiralhanae-led reinforcement fleet sent to the besieged city by the Prophet of Truth deployed several gold-armored hunters almost immediately after the slipspace event. Following his crash-landing, a "flak" hunter was encountered by Edward Buck near Tayari Plaza while he searched for Veronica Dare's SOIEV. Later on, several gold-armored hunters were part of the forces deployed by the Covenant to the Office of Naval Intelligence's Alpha Site, though they were ultimately killed by Dutch and Mickey. Several hours following the initial battle, gold hunters patrolled the deserted and darkened streets of New Mombasa before the city's eventual glassing.
"Flak" hunters appear only in Halo 3: ODST as special hunter enemies in the campaign and Firefight. Regular "blue" hunters appear in the game, behaving similarly to their Halo 2 and Halo 3 counterparts by firing long streams of incendiary gel at enemies while "flak" hunters fire single bolts more similar to their Halo: Combat Evolved or later Halo: Reach etc counterparts. One appears in the level Tayari Plaza in the final section before the player finished the level alongside a regular hunter, and later again during the inner courtyard on ONI Alpha Site. They appear as enemies wandering the streets of New Mombasa in the Mombasa Streets level.
Aside from their unique appearance and weapon, the "flak" hunters behave identically to their regular counterparts, with no AI or health etc differences.
The so-called "flak" hunter is never named in any official source, in-game or otherwise, and their unique appearance has never been elaborated on or seen outside of Halo 3: ODST. The "flak" hunters' name is derived from the campaign enemy's filename -
List of appearances