Last season on Stranger Things, fans were convinced that the game of Dungeons and Dragons in the epilogue had revealed the new monster for Season 2: the Thesselhydra. Instead, we learned the Demogorgons were back again, but in a slightly different format, running in packs like dogs. Or, as Dustin named them, "Demodogs." But what exactly is the difference between Demodogs and Demogorgons on Stranger Things? And why is it important? Beware: This post contains spoilers for Stranger Things Season 2.
In the original Stranger Things run, the Demogorgon wasn't strictly a "Demogorgon" out of Dungeons and Dragons, but a play on the same concept, leading the boys to call it as such. According to D&D lore:
The creature that came out of the Upside Down is clearly amphibious, but it doesn't have two heads. Instead, it has a face that opens up like a flower and eats their victim's insides.
It was, though, a very tall creature (maybe not quite 18 feet), and it definitely walked upright when it came out of the Upside Down and captured Will, as well as when it broke through to attack Nancy and Jonathan (and Steve), and again at the middle school in the final episode.
It was also working alone — a single, solitary creature that came out of the darkness and the "underworld," which is part of why the boys thought of it as a Demogorgon. (Also, it got Will, just like in the game.)
Let's compare this to what we see in Season 2. The pollywog-type creature that Dustin finds does not initially look like a Demogorgon, with a tiny mouth that is not yet ready to flower open. It is also much more pet-like in the beginning, accepting the safety of a tank to sleep in, answering to a name given ("Dart") and eating the 3 Musketeers bar that Dustin feeds him.
This is why Dustin does not recognize the creature for what it is, because it is different than a Demogorgon. The "Demodog" has adopted pet-like behaviors as a survival technique in this new world.
But, the differences do not end there. Even when Dart reaches full-grown status outside of the Upside Down, he does not reach a point where he walks upright like the original creature did, nor does his torso have the broad shoulders and the smaller hips of a two legged creature. Instead, he remains on all fours, like a dog, with arms and legs that function the same way a dog's do and a back that expects to be held horizontal instead of vertical.
And then, there's the fact that these Demodogs are not solitary creatures. They work in packs, like wolves, or lions. In every instance we see them kill, they are working as a team, not solo. They remain attached to those they've adopted as members of their pack, as we see toward the end of the season, when Dustin and Dart reconnect in the tunnels under Hawkins. That is also a major change from Season 1, where the original Demogorgon would have killed Eleven as quickly as anyone else.
So how? That we don't know yet... it's a question still left unanswered. But it's interesting to note that the Thesselhydra (which still may be the Shadow Monster) was the type that could easily interbreed with other creatures and take on their characteristics.
The Demogorgon larvae that Will spat up in the sink in Season 1 probably grew to full size without being detected. (Because clearly it must have done so between now and then, since the new ones grew so fast.) If it then used a dog (domestic or otherwise) in which to host it's next batch of larvae, that would create the creatures we met this year: the Demodogs.
Maybe we'll learn more in Season 3?