Update - 30 Dec 2021
As unprofessional and childish as this is I thought I'd include what started this project. I was thinking about entering the contest last minute and decided to sleep on it when an idea came to mind in the middle of the night- so like any normal concept artist, I woke up at 3AM and scribbled the monstrosity below on my mobile device so I could remember my idea when I was actually back in the world. The idea is he has a skin/beak coverage to help him blend in with the other world which can open and pull back to release his Demo-mouth.
Inspiration taken from the Demo-Dog and a Vulture!
My overall concept was if a spawn of the upside down claimed a bird it would evolve into something like this, taking on more of the features of what inhabited like the Demo-Dog (which would be used for the textures/colouration of the design as shown with the reference to Daniel Edery's Demo-Dog concept and his mouth should be just as slimey and spit-filled).
Although I'm aware if I had spent more time on this project I could have pushed my designs and shape theory even further but it was such a spur of the moment entry that I'm glad I even got as far as I did!
Thankyou so much everyone and good luck!
Keep being S-T-R-A-N-G-E!