From Halopedia, the Halo wiki
The Forerunner Packmaster is a deleted Promethean construct that was intended to be introduced in Halo 5: Guardians. Due to its name, the Packmaster likely led Promethean Crawlers. It is seemingly equipped with a Z-250 lightrifle. The Packmaster did not make the cut for the game, however, early 3D models indicate that the designs may have later been re-purposed as the Promethean Soldier.