Brain Creature (also known as Vomiting Diglett by some fans) 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.
In the game, there is a path next to it that makes odd noises when stepped on. If Madotsuki travels all the way to the right, then she will find herself with the Thing with the Quivering Jaw. There is a spot on the path where the screen will scroll up if the player holds the up button, showing its full appearance. As it is a decorative NPC, it doesn't react to any of the effects.
Appearance
The creature has one eye that is looking in the top-right corner of the map, and a place for what appears to be white liquid to come out above that. There is another hole with has a large arm coming out of it that overlaps the path the player can go on, with some more white liquid dripping off of that. There are also two legs sprouting out near the ground that the Brain Creature is connected to, and an anger symbol above that, similarly to the Thing with the Quivering Jaw.