I’m raising a hand on this one.
They’re not exactly demogorgon and thessalhydra. Or they may be, but I don’t think so. I think that they’re these other-place beings that, through his knowledge of D&D lore, Mike decides to call by names that he’s harvested from his knowledge of D&D.
I don’t think it’s so much that the Upsidedown and the monsters there are taking their cues from the psychic prodding of the D&D game, not per se. I think that the D&D kids are being kids — which I consider a compliment, by the way, and something to be envied, and it’s not meant to be a put-down — being kids, and making what they see as obvious connections.
They see a monster in the real world; it has characteristics similar to this other monster they know about, the demogorgon, which is an equally real monster to them even though they’ve never seen one, equally real because they’re kids and kids are cool like that; therefore, the monster they encounter is a demogorgon.
It’s not a demogorgon because they thought about demogorgon’s and conjured one up; it’s a demogorgon because it was already a monster, it already had demogorgon characteristics, and they called it a demogorgon because that’s what they saw.
That said, you made me think an interesting thought above when you hypothesized about what the Upsidedown is, and said there’s a fan theory about how Eleven brought the Upsidedown into existence.
See, I’ve always been discomfited by the idea of alternate dimensions that we can make sense of. See, it makes me hazy to think that an alternate dimension where monsters are real is also an alternate dimension where perception works the same way. And where biology works the same way. And where…well, you get it. Basically, the Upsidedown is something I have to suspend my disbelief about, because I have to admit that these rules don’t apply, but these rules do, even though the rules that still do apply would, hypothetically, make it impossible for the new rules to work too. We live in a pretty enclosed system. If you throw off or throw out a rule or two about basic biology or basic physics, it has huge repercussions on every other thing. As a result, it’s easier for me to believe that, if we ever travel to an alternate dimension, it will make literally no sense to us at all, because it will be predicated on rules just enough different that we will be incapable of interfacing with it.
So it’s interesting to think that Eleven might have made the Upsidedown. I agree with your statement that she probably definitely didn’t.
What I wouldn’t be too surprised about is if it turned out that — maybe possibly — she inadvertently introduced the rules for how things from our dimension would interact with and perceive things from the Upsidedown, and, contrariwise, how things from there would perceive and interact with things from here. Her act of breaking through, and being psychic and that, may have created a sort of…psychic membrane thing — lacking vocabulary here — that sort of just created a set of rules by which the Upsidedown and the “Downsideup” could reasonably interact. And her experiences — limited, but subconscious — with the world informed how that psychic membrane formed. So that the veneer of the Upsidedown as accessed by everyone from town after that seemed familiar, since Eleven had inadvertently created a set of Laws of Perception that would be familiar to the members of the town.
Or the explanation for alternate dimension from The Flash TV series might apply, i.e., that different universes vibrate at different frequencies.
I gag a little every time I try to say that. It’s just so…huh?
Alternately, none of the above.