From Halopedia, the Halo wiki
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. The Cavalier design eventually became the basis for the Warden Eternal.