Precursor

The Precursors were mentioned in the Bestiarum as the race preceding and mythologized by the Forerunner, being theoretical "Trans-sentient" beings, having the ability to travel between galaxies and accelerate the evolution of intelligent life. The Precursors achieved a higher level technological advancement than the Forerunner, Tier 0, or (Trans-sentience) 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 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. In this way, Humanity is related to the Precursors. It has been speculated that the Precursors actully were the ones who created the flood. Due to them having the power to acelerate intelligent life, it could be said that the flood were the super-evolved version of a species of parasetic organisms. The consequence of their super accelerated evolution was possibly a celluer break down which resulted in the flood. There's also the theory of the legendary planet-the planet seen at the end of Halo 3- being related to the Precursors in some way. It has been speculated that the planet is one of the "beacons" the Precursors set the Forerunners so they could follow the Precurors on their migration to other galaxies.

Trivia

 * The term Precursor means "one that precedes and indicates the approach of another". The term supports that it surpassed the Forerunner. Also, Precursor in Latin can mean Forerunner, but this is only due to the language's ambiguity.
 * The Precursors were the only civilization to achieve Tier 0 of the Forerunner Technological Achievement Tiers and to exceed the Forerunner in technological advancement.
 * 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.
 * The Precursors played the same part as the Forerunner after them, laying the seeds for another civilization to spring up and dominate the Galaxy.