Brain Creature (known as Vomiting Diglett) is a huge monochromatic monster that can be found in the path between White Desert B and the place of The Thing with the Quivering Jaw.
Its right eye looks like it is being torn out and it is vomiting some white juice across the path. It has two legs and its bottom looks like it is sticking on the ground or grows on the ground.
Pressing the up key from the path will make the screen scroll up to reveal its full appearance.