Gloom is the main antagonist in an early version of Disney/Pixar's 2015 animated feature film, Inside Out. He was going to have been grown bigger as he becomes more important.
He is presumably the dark wave after Anger put the light bulb in, and was destroyed after Sadness removed the light bulb. He is also probably the one who made Riley reject the chance of sliding down the stair railing.
Early concepts of him can be seen in The Art of Inside Out book.
Gallery
Early Gloom concept with an early version of Riley
Another piece of concept art of Gloom
How Gloom would grow throughout the film
