Spoilers ahead for Stranger Things 3. Lest you thought the Mind Flayer was gone for good after Stranger Things 2, think again. Thanks to a horde of exploding rats in Stranger Things 3 (and an equally disgusting mass of human flesh and bones, aka the Flayed), the monster returns in full force. It's seemingly banished (again) by the season finale, but dissecting how it came about may prove useful to the gang (and audiences wondering how the Mind Flayer — or some other Upside Down creature — will manifest in Stranger Things 4).
We know the Mind Flayer was hiding in the basement of the ironworks place because it was cold, dark, and damp down there, and through that it was somehow able to keep its essence "alive" (whatever that means for the creature) even after Eleven closed the gate to the Upside Down in Season 2. That seemed to cut the Mind Flayer off from its power source, but unfortunately, it's not clear yet if it can even die. It doesn't have a solid body that we know of, which is why it always inhabits hosts: first Will, then Billy. We also know the Mind Flayer has the ability to control a massive hive mind, which only makes it more dangerous and hard to defeat.
The show first introduced this concept with the Demo-Dogs, and in Stranger Things 3 we see that with the rats. Rats are intelligent animals that also like to hang out in dark, abandoned spaces, so it makes sense that the Mind Flayer honed in on them as the base ingredient for its monster stew — what's unexplained is how the Mind Flayer was able to get a ton of them under its control. If it can send out a signal or infect an entire population of animals through just a few of them, that definitely doesn't bode well for Eleven and the rest of the D&D crew. What's to stop the monster from moving on to larger animals and creating a bigger behemoth? What will Eleven do if it sends several of those giant monsters at once, especially now that she's lost her powers?
As for why the rats and humans started chomping down on fertilizer, cleaning solutions, laundry detergent, and diesel fuel, the most "logical" reason is that the Mind Flayer wanted their bodies to liquefy faster and turn them into what the kids call the Flayed. Consuming any of the above in small doses will give anybody a pretty severe case of poisoning. Repeatedly consuming a cocktail of those substances is deadly and corrosive, literally burning you from the inside out. As Max later says when they find containers of half-consumed cleaning supplies, "If you drink this crap, it'll kill you."
Story-wise, the Mind Flayer doing this to the rats and humans signifies exactly how toxic and backwards it is. The Mind Flayer is from a mirror universe where everything is literally the opposite of what it should be. Someone willingly consuming a ton of poison is a complete perversion of what would normally occur, so of course that's what the Mind Flayer commands them to do. The Flayed look and act just like regular people, but really they're just pureed guts masquerading as who they once were.
It's terrifying, and just goes to show that the Mind Flayer just sees humans as ingredients ready to be killed and controlled. And now that it knows that Eleven is fallible, it's likely only going to get more desperate (and gross) from here. Right at the end of Stranger Things 3, a scene in a Russian prison showed that they already have another Demogorgon walking around, and no intention of stopping their research. Now that the Mind Flayer knows it can get...creative...with organic matter, it's only a matter of time before Stranger Things 4 unveils something new to haunt our nightmares.