Precursor

The Precursors were mentioned in the Bestiarum as the race preceding and mythologized by the Forerunner, being theoretical "Trans-sentient" beings and having the ability to travel between galaxies and accelerate the evolution of intelligent life. The Precursors are believed to have achieved Tier 0 (Transsentience) on the Forerunners' Technological Achievement Scale.

History
From the logs stored in the terminals around The Ark text has been extracted that indicates it is possible that the Forerunner 'Mantle' (Guardians of the Galaxy and all life) was handed down from "legends of the past": the Precursor. It has been presumed that the Precursor viewed the Forerunner as their own legacy, continuing their work after they departed, much in the same way Forerunner viewed humanity before the activation of the rings.

Any further knowledge of the Precursors or their eventual fate is unknown.

Speculation
Several theories exist about the Precursor's role in the Forerunner's lives. 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 achieving inter-galactic travel. No evidence or even rationale has been put forward to verify these claims. As said the Precursor's reached Tier 0, that would mean they could change between an energy form of pure energy some might say but also change to a solid form such as a human but not change into a human but whatever their species looked like. Some can say that they die and come back in an engery form. A Precursor could be the pre pre human form. They died and left the Forerunners who made the rings and found the flood or the flood found them. Last were the humans and surely enough they seemed stronger than forerunners almost able to destroy the flood and survive were the Forerunners failed. I think that The newer the generation the better it would become.