List of known Covenant heretics
From Halopedia, the Halo wiki
Summary
The Heretics are a group of Grunts and Elites who believed in the words of the Heretic Leader and joined his cause. Probably due to the Heretic Leader's belief in the words of 343 Guilty Spark, the Sentinels work with them against the Arbiter and his forces.
The Arbiter and the SpecOp forces led by "Ultra" "Half-Jaw" slew large numbers of Heretic forces on their quest to find the Heretic Leader.
Combat
Heretic Grunts wield only Needlers. They have never been seen using Plasma Pistols, the weapon more commonly used by their regular, non-Heretic counterparts, although a fair amount of Plasma Pistols are scattered throughout their base of operations.
Heretic Elites seem to use either Carbines or Sentinel Beams. Apart from the Heretic Leader himself, they never use the Plasma Rifles wielded more oftenly by their regular counterparts.