Precursor

The Precursors were mentioned in the Bestiarum as the race preceding and might be hologized 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 Forerunner lifeworkers (including the greatest lifeworker: the Librarian) 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.

Precursors are described as "very large", with 4 upper limbs and 2 legs. Also having an "ugly head shaped like arthropods" - like spiders or crabs - they have oval, faceted slanted eyes and a "flat face". Coming from the back of the head is a long tail with a 2 meter barb coming out of it. Whether this is the way all Precursors look has yet to be revealed, but this is the way the prisoner on a Pre-Human world is described.

It is also revealed that this prisoner is the last known Precursor, and that every other Precursor had been ruthlessly killed in a Forerunner-Precursor war almost 150,000 years before the events of Halo and that their answer is at hand - implying that a Proto-Flood form (described as a fine dust, contained in hundreds of glass cylinders) were created as revenge or as a failsafe.

"We meet again, young one. I am the last of those that gave you breath and shape and form, millions of years ago.

''I am the last of those your kind rose up against and ruthlessly destroyed. ''

I am the last Precursor.

And our answer is at hand."

While this is official canon, as the book was written with assistance from 343 industries and Bungie Studios, it would seem to contradict the account of events as described by the Terminals from Halo 3. For in Halo: Cryptum it is discovered that the Didact spoke with this last Precursor. Yet in the Terminals the Didact is answering the Librarian saying that the mantle has not failed them (he is referencing the Forerunners, his own species). He further says that "We can halt this thing!" (Note that he is referring to the flood) "And follow in Their footsteps." Since he is talking about the forerunners inheriting the mantle from the Precursors, and following in their footsteps in that manner, it would conflict with the canon in the book. Therefore it may be inferred that the Flood were created as a failsafe, but slipped beyond Precursor control.

Trivia

 * Many believe that they will be the enemy in Halo 4.
 * 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.
 * There is a possibility that the Precursors may have looked like or been what we call sea scorpions or eurypterids, as described by the Didact in the book Halo: Cryptum.
 * The Precursors' concept of Neural Physics is very similar to the Mass Effect Fields or Dark Energy in mass Effect and/or the Jedi's belief of The Force in the Star Wars universe. Overall is the belief of "Mind over Matter" through science.
 * There is a possibility (according to the prisoner) the Flood was created by the Precursors, and would explain plausible extragalactic origins from a possible long term Precursor fail-safe plan, and not from an intergalactic Flood colony. If this is true, then it means that none of the Flood colonies in the Halo series have ever progressed past the interplanetary stage of development.