Forerunner Cavalier

The Forerunner Cavalier is a deleted Promethean construct that was intended to be introduced in Halo 5: Guardians. The Cavalier was apparently intended to start out as an empty shell, which would then be filled by the Cavalier itself. The Cavalier was intended to use a Forerunner polearm weapon and was capable of shielding itself.

Trivia
It is possible that due to similarities in abilities and appearance, the Cavalier design eventually became the basis for the Warden Eternal. The possibility of the Cavalier being the early version of the Warden Eternal is further reinforced by the Warden Eternal having his own theme music named Cavalier.