There's a lot they could have done. In fact here's my interpretation:
View attachment 118858
Type: Water Poison
Story:
When brought to the Alola region, Grimmer and Muk slowly began to change. The fresh air, and lack of burning of fossil fuels cause a change in their appearance and gene structure. The two, began to be cleansed of their toxic sludge, however a problem arose.
Something went wrong with their genetics and they began to react badly to the purification. Parts of their sludge slightly remained, and the core began to change. Grimer slowly became corrupted and parts of it body began oozing a strange yellow substance. To protect the environment, the people of Alola pushed back the creatures into caves. This way they wouldn't be able to cause as much damage. Wild Pokemon adapted to this and learned to coexist with them.
In order to see in the darkness of caves, the grimer's eyes somehow changed to a bright red and they were able to light up the darkness. They live in peace in caves and are said to still be playful unless made angry.
Researchers theorize that perhaps if their bond with a trainer is strong enough, they can overcome the corruption and live in a stable manner for the rest of their days.
This look so good now! Everything is ordered nicely and really well designed!
This leakers kind of scummy.
He was like "Fine, have some pity, take a page. You'll get the rest when I feel like it." Don't see why he couldn't have leaked it all at once.
There's been a few others, such as ilulmious being a ledbya evolution, or shellos evolves into lapras, but those were very minor, a bigger one was when he fell for this convincing fake during the BW era:
Yes, but knowing that, the guy could make up info that seems convincing. Don't see wht he would though, but it's also strange to not post the other pages.