22,000 BCE - The Manhunter's inherent anti-oan flaw leads them to rebel against their creators and attack Oa. They are decommissioned.
51 years ago - Manhunters tech is discovered by the Psions. Their meddling reactivates the Manhunters who assault Sector 666. The Guardians of Oa instruct the Green Lantern Corps to try to stop them, beginning the Manhunter Wars.
45 years ago - The Manhunters war with the Green Lantern Corps leads to the destruction of the planet Ryut, the home of Atros.
43 years ago - The Manhunters lose their war with the Green Lantern Corps and are finally shut down by the Guardians of Oa.
27 years ago - The Psions construct a new model of Manhunters to serve as a police force in the Citadel after the Darkstars are destroyed by Ferrin Colos.
10 years ago - The Manhunters destroy the planet Xanshi and frame Green Lantern John Stewart. The princess Yrra Cynril is the lone survivor, fleeing to deep space. The Guardians of Oa are forced to place him on trial in Citadel court where he proves his innocence, prompting the Manhunters to attack Oa.
There are very few comic book concepts more universally useful than 'evil robot army'. You see it crop up in superhero movies all the time; it gives you endless waves of things to punch while you work your way toward the central thing-that-stops-the-robots. They're practically a story arc within themselves. The fact that the Manhunters provide you with this tried and true method of story building while also having a pretty cool backstory that ties into the larger mythology of the hero makes them a very useful inclusion in our timeline.
The Manhunters Comic History
The Manhunters weren't created specifically as a Green Lantern enemy, they just used the mythology of Green Lantern in their backstory. They debuted in Justice League of America #140 in 1977, the previous creation of the Guardians of Oa. It's worth noting here that the name Manhunter has been around in DC for much longer, applying to quite a few different characters over the years (some of whom actually appear in our timeline). The look and color scheme of the Manhunters actually come from the original Golden Age Manhunter, Paul Kirk. Martian Manhunter himself is an obvious reference to this legacy name.
After the initial appearance of the Manhunters, their major contribution to DC's story was the Millenium event. Dozens of characters across the entire line were revealed to be Manhunter robots in disguise. It's very much DC's version of a Skrull invasion. It wasn't the best crossover event but if you were reading comics in 1988, you sure as hell knew about it.
Our Manhunters Story
The idea of an ancient robotic police force created by the Guardians of Oa is a good concept, and it's been approached several times over the years. The ancient history of the Guardians and of the DC Universe overall gets altered quite a bit from year to year, so there really isn't one MAIN version that we're going to draw from. Instead we're going to zero in on two core ideas that will define how we use them.
The first idea is the existence of the ancient Manhunters, the robot police that were build thousands of years ago before the Green Lantern Corps ever existed. We're using the story of Krona to explain the existence of 'anti-oan energy'... literally just the concept of entropy, that all things that exist will eventually not exist. This anti-oan flaw would exist in the Manhunters programming, leading them to eventually revolt against their creators. These ancient Manhunters would later be discovered and reactivated by alien creeps the Psions. This leads to the Manhunter Wars which happen in relatively recent history, involve many of the current Lanterns, and create the eventual conflict with Atrocitus.
Separately, there are also the modern Manhunters built to police Citadel space by the Psions after the destruction of the Darkstars. This allows us to have modern appearances of the Manhunters without having to create a story of all-out intergalactic war every time. They can function more as a rival intergalactic police force to the Lanterns, which should create some cool conflict. In the Justice League Animated Series the Manhunters were depicted arresting John Stewart and taking him to Citadel space to stand trial for the destruction of a planet, and that story would actually play really well. This of course leads to the Manhunters again attacking Oa, but this time the Earth Lanterns like John, Hal and Guy will actually be in the battle.
As we leave our timeline, the last appearance of the Manhunters is back in the lost sector where the earlier Manhunter Wars took place. Since then, that sector has been closed off from the Green Lanterns, and in it's place Atrocitus has become a despot. Both Hal and Kyle have had adventures there. If Atrocitus finds lost Ancient Manhunter tech and begins plotting to use them, it sets up a possible story in the future where Atrocitus and his Red Lanterns actually leave his sector and attack the universe at large. Having modern enemies find and repurpose the Manhunters is a pretty regular occurrence in the comics, and with good reason, because it works really well.