Users are or can either embody or transform into an Id (also known as the Shadow Archetype/Aspect); is the Freudian representation of uncoordinated instinctual trends in human behavior known as the subconscious of one's mind. 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 Shadow is a negative aspect that the individual must get along with. In addition to that, the more it gets repressed and ignored, the more powerful it becomes. Shadows can be considered horrific, inspiring fear in the person who's been facing it.
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.
Inner Jocks (Channel Awesome) are the parts of one's mind that focus less on the intellectual sides of movies and more on simple appeals, preferring what is "awesome" like action, gore and badass things.
Sindy Carmack (Loki IRL) is a living Id that finds great amusement in influencing people and IRLs alike to do bad and selfish things.