Demodog

Demodogs are apparently a subspecies of demogorgons.

History
After Will Byers returned from Upside Down, he apparently came along with a parasite, which for some unknown reason instead of keeping that parasite inside, he spat it out in his bathroom sink when he returned home, causing the parasite to flee down the drain. Later somehow instead of becoming a demogorgon (which probably should have happened) he became a different species called a demodog, and while in his larval stage he ended up in Dustin Henderson dump in Hawkins on Earth and when found to be what appeared to be a tadpole, it was nicknamed D'artagnan, he was the first existing demodog. After its complete growth, several other demodogs emerged, with Mind Flayer being the one who controls all demodogs, after Eleven sent most of the Mind Flayer from vollta to Upside Down and closed the portal, the demodog's fundamental link with Upside Down was cut, causing all who remained on Earth to die.

Biology
Demodogs work like a beehive mind, where Mind Flayer would be the “queen” and demogorgons would be workers and demodogs would be soldiers, demodogs have no apparent weakness like the demogorgon had. Demodog minds are not as susceptible to Mind Flayer control, as the demogorgon obeyed all of his orders while Dustin's demodog, D'artagnan did not attack him after remembering him. They can also eat chocolate, which indicates that they can some fruits, milk and sugar.

Slug
While in stage one, the demodogs look very black larvae, appearing to be dirty, after a while parasitizing a living being it reaches stage two, which seems to take a while since there was a gap of some years between Will's spit in the sink and find it in the trash.

Pollywog
It logically gets bigger and creates a tail, takes on a more greenish color and two forelegs, has no eyes, nose or ears, but has a very small mouth to feed on.

Frogogorgon
At this stage he grows a little more and two hind legs "jump" out of his body, he takes on a more greenish color and is about the size of a frog.

Catogorgon
In stage four it starts to get more interesting, he sheds his skin (similar to what snakes do) looking like a demogorgon, having his legs more developed, he grows in size getting a little bigger than a cat, he develops a neck, and its head remains without eyes, ears or nose, however, its mouth grows much more and now it has the capacity to open in several parts, being in the shape of a star when opened, with several rows of teeth in each corner of the mouth.

Demodog
It grows much more, getting to be compared to larger animals like wolves, but otherwise its appearance remains the same, only decreasing the size of the tail.

Culture
Demodog serve only and exclusively to fulfill the desires of Mind Flayer, an entity of the Upside Down dimension, they are probably the "soldiers" of the "hive", since they are much faster and in greater quantity than demogorgon. Demodogs begin to really serve Mind Flayer when they reach sufficient maturity in stage five, where they become aggressive and attack everything in their path under the orders of Mind Flayer, but there is still the possibility of domestication, since Dustin did this by taking care of his demodog when he was still in stage two, making it possible for them to get closer and making him remember Dustin when he started to be controlled by Mind Flayer. Demodog always roam in large groups, usually having dozens of individuals other than normal terrestrial packs.