Demogorgon.

Sean Carter

Here is my Demogorgon costume from the show Stranger Things. Made with a spandex body suit with insulation foam. 
Mask made from wire, duct tape, air clay. 

Alex Mercer

As a Stranger Things fan, I approve on this. I really like the creativity and I commend the dedication you put into this. 

In case you'll be needing some equipments in creating such costume, Stan Winston Kits are a very good choice. Here's the site. I do hope this helps.