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.

Speculation suggests that the Precursors may have created the Flood to lead the Forerunners into their discovery of Humanity, though this is unlikely, due to the fact that the Precursors left the Forerunners their "Mantle", and killing their descendants wouldn't make much sense. Also speculated is that the Precursors created the Flood as a means of biological weaponry against another species, but the Flood developed intelligence and became self-aware, eventually becoming able to spread inter-galactically. Though this is also unlikely, as the Precursors would certainly have the means to disable the Flood.

Their are no recorded historical references to the Precursors other than that they left their "Mantle" (Secured peace and biological freedom in the galaxy) to the Forerunners. Whether or not they are extinct, or simply somewhere else, there is no evidence to support either. More theories suggest that if the Precursors left, they probably lacked major resources. Perhaps, as speculated, they breathed a rare gas that was now abundant in the Milky Way, and therefore had to leave. This would connect the theory that the Precursors created humanity. When the Precursors left, they had to leave behind a guard of the galaxy, and found the Forerunners, who could still live off of resources in the galaxy. Since the Precursors could accelerate the evolution of intelligent life, the Forerunners almost instantly became hyper-intelligent sentient biengs, but with no knowledge of their possibly stone aged past. The Precursors, with no time or way to explain, created humanity for the Forerunners to discover, and learn about their past, before leaving.

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.