Wendigo/Alien (Plus Welcome To Derry spoilers)
Long time lurker, not really a poster. Hate to fall into the pool of 500 theories but... nothing really ground breaking here, just connecting some dots I don't think I've seen connected before (If they have I'm sorry)
It just randomly hit me that I dont think we've seen the monsters actually eating anyone, just the aftermath. We have, however, seen the MIY eating organs. So it appears me to that the towns folk monsters work for the MIY, and dont kill for themselves at all, but rather to bring him the organs. I know that's been "established" that the MIY commands the monsters but this to me solidified it. Now that we know they become reborn, there would be no technical reason for them to "eat" or do anything to keep themselves alive. Which may be why MIY is banking so hard on another massacre, and why things will most likely get interesting when Jade and/or Julie's progression in their knowledge will inevitably interfere with that event. Prolonging or avoiding it may be what kills the MIY.
In connection to that, it made me wonder why they are always smiling, taunting, and stalking their prey if they dont actually need them for sustenance, and why there appears to be "cycles." My mind automatically went to Pennywise as silly as that sounds. But...Pennywise used these same tactics to raise fear and adrenaline because it made the meat taste better. And, (if anyone watched Welcome To Derry) we learn that the culmination of every Pennywise cycle ends with a giant massacre or "feeding event" usually brought on by the townsfolk turning on each other by strife Pennywise created, or simpler things like posoining the water supply. (Which was also hinted at by Saras water test)
I don't think the writers would straight up rip off Stephen King but hard to deny the similarities.
My second theory, as to what they could be if not alien, is a wendigo. Wendigo, in folktales, is a cursed human that transforms. And, in some instances, much like vampires, victims can become possessed by wendigos, becoming one themselves. If anyone has played the game Until Dawn, or just googled wendigos, there are alot of variations. The more traditional looks like a combination of a dead human body with a deer skull for a head. The other, however, is often sullen dark eyes, bald or stringy hair, and sharp teeth, which are SUPER similar to the monsters true forms in this show.
I have no answers for the town itself or the overarching story here. Maybe this will go nowhere, or maybe it will spark some thoughts.