Precursor

The Precursors were mentioned in the Beastarium as the race preceding and mythologized by the Forerunner, being theoretical "Trans sentient" beings and having the ability to travel across the galaxy 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.

Whether the Precursors died or simply left this galaxy by some other means 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.