Users become the embodiment of the Id; the Freudian representation of uncoordinated instinctual trends in human behavior. In other words, the unfiltered wants and desires within the mind. In Sigmund Freud's theory, the Id and the Superego (the part of the mind that places foreign factors like other people into consideration) inserts suggestions to the Ego (one's own sense of identity). While the superego demands what the person would consider the "right thing" to do, the Id demands what the person wants to do despite the consequences. Because of this, people consider the Id's influence to be a bad influence and is often associated with evil.
The Mask (The Mask) is an object forged by Loki that empowers the user with Loki's power, but removes all internal masks, making them into what they truly are on the inside.
Among the many jobs that Demons (Your Pretty Face is Going to Hell) have, they also have the job of "Shoulder Devil", leading living humans to commit evil acts of varying degrees subconsciously.
When given a moral situation, Kronk (The Emperor's New Groove) is given two options by his shoulder angel and shoulder devil. The Shoulder Devil always recommends the easy way.
When exploring Rusty's mind, Orpheus encounters his Id (The Venture Bros.), his hedonistic side.
While exploring Duck Dodger's mind, the team comes across his Id (Duck Dodgers). Because Dodgers has little to no self control, his Id is left underdeveloped enough to where it looks and acts as a Neanderthal, too dumb to spell his own name right.
Nightmare (Ultimate Spider-man) disguises himself as Spider-man's shoulder-devil to convince him to quit being Spider-man.
Langdon Cobb (Futurama) is a part of a species of living ids, their egos taking on seperate life as a fungus-like dog creature.
In order to free himself of his writer's block, Kevin Thorn summons Old Sam and Hansel (Fables) to act as his two respective muses...
...Hansel encouraging him to rush through his writer's block and undo the universe all the faster.
The Little Ogre (Soul Eater) is the manifestation of the black blood in Soul's mind, goading him to use the madness with the risk of his sanity.