Precursor

The Precursors were mentioned in the Bestiarum as the race preceding and mythologized by the Forerunner, being theoretical "Transsentient" beings, having the ability to travel among galaxies and accelerate the evolution of intelligent life. The Precursors achieved a higher level technological advancement than the Forerunners, being Tier 0, or Transsentience on the Forerunners' Technological Achievement Scale.

History
Not much is known about them, but information that can be found by text from the logs stored in the Terminals around the Ark indicates it is possible that the Forerunner 'Mantle' (Guardians of the Galaxy and all life) was handed down from "legends of the past": the Precursors. It has been presumed that the Precursors viewed the Forerunners as their own legacy, continuing their work after they departed, much in the same way Forerunners viewed Humanity before the activation of the rings. Having obtained the status of Tier-0, it is likely that the Precursors can travel between galaxies.

Trivia

 * The term Precursor means "one that precedes and indicates the approach of another." The term supports that it surpassed the Forerunner.
 * The Precursors were the only known civilization to achieve Tier 0 of the Forerunner Technological Achievement Tiers and to exceed the Forerunner in technological advancement.
 * The Latin root word "trans" means "across" or "beyond." "Sentience" is the ability to experience sensation, and is often used to imply sapience, the ability to think.  A transsentient being may thus be considered beyond any recognized being and be on the level of godhood.
 * In Halo: The Fall of Reach, Dr. Halsey uses the term Precursor to describe the alien race that created the crystal found on Sigma Octanus IV.