Appearance Is in the Eye of the Beholder

Some characters Lie to the Beholder, appearing selectively different to one or two people than they appear to anyone else.

Other characters appear differently to anybody who sees them. One man will view them as a tall blonde woman, where another will see a short black man. Obviously, this makes tracking the character down by description difficult. Such an ability is almost always mystical in nature, although scientific or pseudoscientific explanations like "psychic resonance" or "nanotechnology" are often used in science-fiction settings. Keep in mind that when Appearance Is in the Eye of the Beholder, that means that the entity involved looks different to multiple people at the same time.

A common variant of this in settings with multiple species is to have an entity which appears different depending on the species of the beholder, most often as a member of the same species.

A Form You Are Comfortable With and You Cannot Grasp the True Form are often forms of this. Compare Lie to the Beholder, which generally causes the same illusion to all affected viewers. Compare an Empathic Shapeshifter, who physically changes, while this trope is more often an illusion. Often employed by a Master of Illusion

Comic Books

 * This is why Galactus looks to us humans like a human with a funny hat and the letter G on his belt buckle. To the Stone Men from Saturn he looks like a Stone Man, to Toad Men he looks like a Toad Man, etc.
 * Thanks to Word of God, this could describe how in Calvin and Hobbes, Calvin sees Hobbes differently from everyone else.
 * This is explicitly shown several time in The Sandman, where Morpheus (as his name implies) always appears like a member of the observer's own species and culture, or like what the observer would expect the God of Dreams to resemble.

Literature

 * In Stephen King's IT, when the heroes go to the villain's lair, the sign above the door of the lair is like this: each character sees it differently.
 * In the Doctor Who New Adventures novel Sky Pirates! the Doctor himself was heavily implied to be one of these.
 * In The Wonderful Wizard of Oz, each character is asked to see the Wizard one at a time, appearing as something different to each one. It's all a trick, however; the real "wizard" is The Man Behind the Curtain.
 * There's an interesting variation in the Dragaera series: the goddess Verra is not described as appearing differently to everyone who beholds her, but it becomes clear that when she speaks, everyone hears something different, effectively holding multiple different conversations simultaneously. Descriptions of godhood elsewhere in the series imply that this is a result of one of the defining characteristics of deities and demons in the Dragaera-verse.
 * In The End of Mr. Y, everything in the Troposphere is dependant on the eye of the beholder.
 * In The Well of Lost Plots, the Grand Panjandarum resembles the beholder when they appear at the end of the book.
 * In the Young Wizards series the Powers That Be can manifest in multiple ways. In one particular form of manifestation they look different to every mortal who sees them. The main character perceives the Lone Power to be a handsome faced man with red hair, and the Arch Angel Michael to be a glowing seven foot tall woman wearing sweats and sneakers.

Live Action Television

 * In Babylon 5, the character Kosh (once outside of his encounter suit) appears
 * Pleasure GELFs in Red Dwarf can appear to any onlooker as their perfect mate, despite looking like disgusting green blobs in their natural form.
 * Also Psirens, whose true form was some sort of giant beetle thing.
 * Star Trek: The Original Series episode "The Man Trap". Early in the episode the salt vampire made itself appear differently to three different people simultaneously. It looked like a young Nancy Crater to McCoy, an older Nancy Crater to Kirk, and to crewman Darnell it was a young blonde woman who looked nothing like Nancy Crater. Later in the episode it changed methods, appearing to all comers as an the older Nancy Crater, and making use of a different Trope

Tabletop Games

 * Any member of the Romancer Kith from Changeling: The Lost always appears as the beholder's perfect ideal of beauty.
 * The Obfuscate Discipline power "The Familiar Stranger" from Vampire: The Requiem has an effect somewhere between this and The Nondescript.
 * The monstrous dream larva in Dungeons and Dragons uses this ability to be every one of its enemies' worst nightmares at once.

Video Games

 * A background conversation in Mass Effect 2 suggests that the asari may do this.
 * This is used as a handwave in the bonus chapter of The World Ends With You, explaining why a character who logically should look different in this timeline still has the same appearance--Neku knows the character one way, and is incapable of perceiving otherwise. (When he challenges this, the real explanation breaks the fourth wall: why bother making a new appearance for a character who will only show up once?)
 * Kingdom Hearts: 358/2 Days has this occur with ., although the dialogue encourages people on their first read-through not to pay much mind to it.
 * Shin Megami Tensei: Implied with Lucifer. Since he rarely appears to more than one charcter at a time and the player share's the main character's POV, it's easy to write his different appearances off as Shapeshifting or Depending on the Artist. However, he casually mentions that the Mushibito call him "Beelzeboo" because they see him as a fly.

Web Comics

 * The Kevin and Kell version of the story of Jesus' birth has the baby Jesus seen as being the same species of whoever looks at him.

Web Original

 * The Slender Man had this effect in the earliest stories. These days, he's more often depicted as The Blank.
 * A video from College Humor features an "Optical Illusion Girlfriend" who looks like a pretty girl to her boyfriend but an ugly hag to his friend. After an argument with his friend over her ambiguous driver license picture and them both trying to explain to each other just what they see in it, the boyfriend's perspective of her suddenly switches to the "ugly hag" and he flees in horror whereas his friend begins seeing the "pretty girl" instead and approaches her with a leer.

Western Animation

 * The Weird Sisters of Gargoyles all take the same form at the same time, appearing as a trio of creepy little girls, old crones, aged female gargoyles, or voluptuous young 20-somethings, as befits who they are speaking to at the time, though they can still be told apart apart by hair color (blonde, black, white). The little girls are seen by the Manhattan clan, the old crones are seen by Macbeth, and the old gargoyle hags are seen by Demona. Meanwhile the 20-somethings are their preferred form, seen by the audience and other Children of Oberon as well as any characters not implied to see them differently (although humans will see them in period/job appropriate attire). Word of God has stated that only the Third Race and the audience ever see them for what they truly are.