Most accounts seem to point out that the first Harbingers appeared at around the Seraphic Age, back when the Angels ruled divine dictatorships on Bumi. They believed that Sol's Order should not be rivaled by Luna's chaos and sought to forcibly convert all other sentient races towards absolute obedience and worship towards
The Angels had a massive, seemingly unstoppable army at her disposal and even though the Infernals had successfully brought down numerous divine dictatorships in the past, it would seem that nobody could stand in their way.
But when all hope seemed lost, a loud crack was heard from the heavens followed by a brief period of impermeable darkness. After that darkness cleared, several humanoid figures had appeared as if clothed by the same darkness that had briefly swallowed the world. Their appearances stroke fear and despair to all who gazed upon them. Their faces were vaguely human, a perpetual grin affixed to them signifying that they were creatures of chaos and madness by nature.
With their arrival signalling the end of the tyranny Lucifina imposed not only on Celestia but on Bumi as well, the Infernals aptly named this new race as "Harbingers".
However as much as this new demonic race was revered as heroes, they were also feared for their abilities. If their presence alone could determine such a grand battle between the Celestials and Infernals, then who's to say that they would not try to rule the world themselves? As a result, attempts were made to hide their existence from the world.
The suppression of information about the Harbingers followed, and soon their existence was turned into nothing more than the stuff of legends, but the Harbingers never truly disappeared and while the days of the Seraphic Era are long gone, the Harbingers remain, awaiting the time when they would be needed again.
Potentionally the harbinger are a bastarized variant of another species to allow for a better word, or they do better as dopplegangers to have a base template, lets say with there own they copy the apperance dependant on age of the same sex parent, but if they mix with another infernal race it always results in twins, one of whatever race and of the harbinger, with the child picking up the appearance of the other twin to add to the fold. It could be how they breed in general but that might leave them too dependant on other races. Either way it would make them popular choices for those in high politics who needed look alikes or for those who wanted a large family
Credits to Accelerator