Primordial races have to be a little weird, so just saying "freaky megafauna but intelligent and mildly humanoid" doesn't quite cut it.
Again, I offer up my idea of a Primordial race that reproduce by sacrificing one of their own positive Intimacies and were once considered friends to mankind - because humans barely live any time at all, so you can just enjoy them while they're around, then carve out your feelings of attachment to them once they die to make a child. All the good parts of having a friend/sort-of-pet, none of the grief!
They reacted
very poorly when a sizable component of humanity suddenly started trying to tear down the Primordials and were expecting their aid in doing so - in much the same way many people would react poorly to their pet tarantulas suddenly trying to enlist them in a global conspiracy to unseat humanity as the dominant species on Earth.
Almost all of them were exterminated, with a few survivors who fled into the Underworld or got dragged into the Rebellion on humanity's side through misplaced affection, terror of being killed for refusing, or Exalted social mastery.
The Underworld-dwellers degenerated into horrific living Sin-Eaters, their means of propagation twisting so they now needed to harvest positive Passions from the mortal dead (thus damning them to become ever more stained in necrotic Essence with each generation, and ever more bitter over their fall from grace). By the Age of Sorrows, their horrifically degenerate progeny had claimed a web of fractious city-states in the depths of the Underworld, ruling through the trade of soulsteel, ghostly slaves, and what resources can be extracted from the Labyrinth (whose corruptive influence they enjoy a slight resistance to, in comparison to most ex-human explorers - although their harvesting operations still suffer stiff attrition rates, which further drive their need for stolen Passions.)
Many of those who chose to serve the Incarnae died in the war, and others were killed by Exalts outraged at the perceived exploitation of their ancestors. Efforts to repopulate left many of those who remained bitter & despondent, their hopes and dreams burnt to birth the next generation - who knew nothing of the world that their parents saw die, and often drifted away from them to become surrogate children of more familial Deliberative members. By the time of the Usurpation, their original culture was all but gone, with only a tiny handful of wizened elders even recalling secondhand accounts of their forebears (and largely exiled to reservations where they were kept as patronizing reminders of the "savage prehistory" of the world; the rest were long since assimilated into the new order, eagerly serving as nursemaids, assistants, majordomos, and test subjects for their Exalted masters.