Precursor

The Precursors were mentioned in the Beastarium as the race preceding and mythologized by the Forerunner, being theoretical "Transsentient" beings. The Precursors are believed to have achieved Tier 0 (Transsentience) on the Forerunners' Technological Achievement Scale.

From the logs stored in the terminals around The Ark, it is possible that the Forerunner 'Mantle' (Guardians of the Galaxy and all life?) was handed down from "legends of the past", which can possibly have been the Precursors. It is also possible that the Precursors saw on the Forerunners as their inheritors when they left the galaxy, much in the same way as the Forerunners saw humans as their children and inheritors.