Precursor

The Precursors were mentioned in the Bestiarum as the race preceding and mythologized by the Forerunners, 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 of tier one technological advancement than the Forerunners, so tier zero technology is attained, (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 Forerunners 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. Any further knowledge concerning the Precursors or their eventual fate is unknown.

Because the stone structures found on Delta Halo do not seem like they were built by the Forerunners, (The Forerunners, as far as we can tell, only built with metal) it may be possible that the structures were built by the Precursors. Also, Cortana noted that the stone structures were "sacred" and "protected". This is cause to believe that they were built by the Precursors and moved to Delta Halo.

Trivia

 * The term Precursor means "one that precedes and indicates the approach of another". The term supports that it surpassed the Forerunners.
 * The Precursors were the only civilization to achieve Tier 0 of the Forerunner Technological Advancement Scale and to exceed the Forerunners in technological advancement.
 * Precursor in Latin can mean Forerunner, but this is only due to the language's ambiguity.
 * 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.