Similar to Vanilluxe, I wanted to make a Nega-Mega Evolution of Garbodor partly because none of the Gen V pokémon received real Mega Evolutions, and partly because it's one of the most despised pokémon in the fandom. However, I admit that thinking of a design for this one was more difficult.
I was basically trying to go for a pokémon made out of garbage that, like monsters such as the blob, becomes bigger whenever it eats more, and I wanted to have a a few environmental references that I thought would work with it.
I based Nega-Mega Garbodor's body style off of No-Face from Spirited Away since it had the similar effect in the movie when it was eating everything in sight (also referenced in its Moxie ability), while the yellow eyes and the shape of its hands from its garbage bag mask are based on The Once-ler from Dr. Seuss' The Lorax (the book, not the movie) since Seuss is one of my biggest artistic inspirations.
I also took the two clump appendages on top of Garbodor's head and turned them into little chimneys that emit smoke (similar to ones found in factories), turned both of its arms into matching pairs of legs (which could also be used as hands), as well as having acid drip out of its nozzle-like fingers. For color choices, I tried to use darker tones of Garbodor’s regular color scheme, while adding green lumps to the pink and blue ones to reference the colors from the Gen I games