Short answer: it's likely a custom monster, because it doesn't fit neatly into any pre-existing categorization in any other game line.
As described, a devourer exists strictly in the physical, carnal world, it's literally a black mist hanging in the air. That rules out any kind of ephemeral being: it's not a ghost, a spirit, an angel of the God-Machine, or any class of demon from Inferno, all of which are ephemeral entities that can exist in Twilight, while a devourer never does.
Devourers don't possess people like ephemeral beings, possession is strictly nonphysical, with the entity remaining in Twilight superimposed on the host. Devourers instead physically install themselves in the body, visible as a "phosphorescent lattice of circuits lurking beneath the host’s skin" and physically puppeteer the body technologically.
Devourers do have a few similarities to the shadow-birds, the Strix, to a certain extent. Strix are non-ephemeral, are made of 'shadow-stuff,' and possess hosts by invading their airways. But there are some key discrepancies: devourers are pure hedonists for pleasure and pain with no long-term memory, while Strix have a wide variety of motives, many with devious long-term plans. Powerful Strix can possess a living human, but torturing the host to the brink of death--'mortification of the flesh'-- can't exorcise a Strix, which can simply continue to possess the resulting corpse instead. Whereas a devourer releases its hosts when caught in the "throes of ecstacy" from the pain. (Strix even devour living flesh to feed, but only when possessing a corpse. They're more limited when possessing the living.)
Devourers share a mechanical motif with imps, and imps are clearly otherwise known as Imperatives from Demon Storyteller's Guide: Rank 1 ephemeral entities, far simpler and less intelligent than an angel, created for some purpose by the God-Machine and now repeating it in an endless routine. (Utopia Now outright mentions this more accurate label for them, 'the “imps” or “imperatives” that repeat tasks like broken records.')
That all hints that devourers might be yet another minor creation of the God-Machine, since they seem to be artificial, inexplicable, and high-tech far beyond known human capabilities. Their description does suggest they had a purpose to their creation:
Their
memories seem non-existent; they are eternally curious about
simple sensations of the flesh. They’re prone to crimes of bizarre
passion. They tend to retain small, ritualistic behaviors, though,
indicative of an original purpose they haven’t fully abandoned.
