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 concerning the Precursors or their eventual fate is unknown.

Trivia

 * The term Precursor means "one that precedes and indicates the approach of another". The term supports that it was surpassed by the Forerunners.
 * A game involving the Precursors might be in the works. There is a game called The Precursors that has space travel. However, it is unconfirmed if this is Halo related.