Big Halo Lore Dump below, linking to things in Infinite:
The portion of Precursors left the Galaxy during their war with the Forerunners and declared they would return. But what they returned as was not Precursor it was instead the extra-galactic parasite we know as the Flood. It originally infested ancient human colonies and drove humanity into Forerunner territory which erupted into the Ancient Human vs. Forerunner war... before the Forerunners found the flood that is... then there really was only 1 option left after all the fighting - ignite the rings.
However, any species catalogued or imprisoned within Cylix's would not be eradicated nor would any sentient beings on the Ark. It is unknown if any Forerunners did this to save themselves but we do know Cryptums also protected their inhabitants from the effects of the Halo array firing.
This all said, who/what are The Endless? They could be something totally new - for example we don't really know much about the race that the Skimmers and Harbinger belong to - are they Endless too? The Harbinger seems to be trying to bring them all back and Atriox seems to have found their imprisonment Cylix's on the ring in the post-credits cutscene. Plus, in the Legendary ending we can hear a conversation from eons ago regarding their imprisonment. However... are they some form of the Precursors? Who can say right now. But it is somewhat hinted at/brought up that either the Endless and/or the Skimmers/Harbinger's race are called "Yalanyn/Xalanyn" in an audio log.
The Precursors we've come across so far in the books (well the one, I guess) have been described very very differently to what the Harbinger and Skimmers look like - however the symbol/shape we see on the Cylix's in the post-credits cutscene is much more nebulous and strange and could be the Forerunner symbol for their species... but a Cylix seems to show the outline or a 'best estimation' design of sorts on the front of the catalogued species and the one we see in the Post-Credits scene does not look very humanoid at the very least.
Update:
I feel like I should also expand that the closest thing to a Precursor in the games is The Gravemind. The Flood is a form of Precursor, they're trying to emass enough intelligent life to form a Gravemind which is akin to one Precursor. But that doesn't mean precursors originally looked like The Flood or Gravemind - they were a power sentient species and weren't parasitic in nature until they left the galaxy and returned. We don't know why they became the flood or how.
We even have
official artwork of a Precursor - and the name Precursor is likely not their actual name/species name. It is most likely a moniker they accepted or chose. However, we do not know if this Precursor is of their original form or changed/altered just like the Gravemind in the games, as we learn this Precursor is technically a Gravemind in the last of the Forerunner Trilogy of books. However in what way is never made explicitly clear.
It is very likely we have not seen a true Precursor before and we may never, unless they and The Endless are one and the same.
