: Maybe it's a covie/forerunner blend for their new gear.

: What would the story rationale be? Are these covies who have been stuck on a
: shield world for a long time?

Without context it's hard to say. It might be some of these are multiplayer shots, hence a larger number of units than in campaign. Presumably this is set post-Halo 5, as we never heard about the Spirit of Fire even after the Escalation tease, and this faction apparently is built up enough to have Brutish designs for their materiel. Or, perhaps they've been given a leg up by the Created?