One of the appeals of Stranger Things is the way it treats its creatures. There are no humanizing aspects to these villains. Like any creepy alien movie of the '80s, the monsters are monsters and the heroes are the heroes. It's the perfect recipe for a quintessential good vs. evil story.
However, what if this is all a façade, and not only is the Demogorgon more humanized, but it's actually one of the protagonists' future form?
Upon a Stranger Things rewatch, Redditor u/CinnaSol cooked up a doozy of a fan theory. What if the Demogorgon is actually Will from the future, sent back in time through the Upside Down to perpetuate a never-ending time loop? It sounds a bit more like Dark than Stranger Things, but it makes sense after a bit of explaining.
Firstly, the Upside Down needs to be explained. We already learned the Upside Down is another universe that the government tore a gate into through scientific experimentation. That's all well and good, but suppose the parallel universe isn't just a dark, twisted, ashen version of Hawkins: it's the future, post-apocalyptic version instead. In other words, the government didn't find a portal to another dimension, they found a portal into the future.
The loop continues as follows: the government lets the Demogorgon in through the gateway, the Demogorgon/Mind Flayer proceeds to methodically take over the world, which then becomes the future world, which then is linked to the past again, and so on and so forth, potentially for eternity.
So how did the original Demogorgon originate? It couldn't be from the Upside Down organically if the Upside Down is just the future, so the answer is someone we know and love from Hawkins. Looking at the suspects, it's easy to point out the obvious answer: Will Byers.
The Demogorgon hunted down Will and knew exactly where to find him, but he wasn't killed —unlike Barb. Instead, he was taken but seemingly unharmed. Why would the Demogorgon do this unless to get his past self out of the way to avoid any time travel paradoxes?
In Stranger Things Season 2, Will explains why the Mind Flayer seemingly wants to kill everyone but him. Why? Because Will is the center of its plan, killing him would cause a simplified version of the grandfather paradox: killing Will would erase the opportunity for Will to ever get access to the Mind Flayer and become the Demogorgon.
When Will is possessed by the Mind Flayer in Season 2, he also describes the visions he sees from the monster's perspective as "memories" that aren't his. It's almost as if he's gaining access to his own future memories, and his body doesn't know how to process the.
There's just one big issue with this theory: it all rests on one big bootstrap paradox. The only way the future world can end up so desolate is through the Will Byers plan, but the Will Byers plan relies on the Demogorgon.
This is a pretty common flaw in time travel stories. When time loops are involved, events can happen without any origin because they occurred in a non-linear fashion.
The Inverse Analysis — Stranger Things thrives in its simplicity, but a complex time loop riddled with difficult to explain paradoxes could take the show from a creepy nostalgia trip to a mystery box prestige show that could give Lost a run for its money. Considering how Stranger Things has done so far, it may not need a time loop ace up its sleeve, but it's interesting to piece together how one is built-in, just in case.