Precursor

The Precursors were mentioned in the Beastarium as the race preceding and mythologized by the Forerunner, being theoretical "Transsentient" beings and having the ability to travel intergalactically and accelerate the evolution of intelligent life. 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 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.

It should be noted that the name and description both suggest that this might be a reference to Star Control II: The Ur-Quan Masters.