Replicant Snatching

So you're a plucky highly advanced cyborg, and you want to bring about the downfall of humanity through propagation of your own kind. Good for you!

Unfortunately, there's a problem. Because you're a physical being, you can't flat-out possess someone. Even if you can shape-shift into a perfect replica of a human, you can't just integrate yourself into normal human society, without any form of identity. Sooner or later you'll get the cops on your tail, and all they need to do is put you through a metal detector and it's Game Over. Even perfectly imitating a real individual will cause problems if the original shows up.

Then you see a Genre Blind Innocent Bystander mooking his way down an alley. Hmm.

The Innocent Bystander's family doesn't notice anything. Why should they? He's the same as he always was. As long as they don't check the one dumpster where you left his skinless corpse and his removed, scanned-for-memories brain, you're safe. In all your plucky advanced cyborg glory.

See also Dead Person Impersonation and You Are Who You Eat.

Anime and Manga

 * How the Akuma of D Gray Man gain their human forms. To add to the horror of it, though, the body they use is always the one who called the soul powering the Akuma back from the dead—which only works if they are someone the deceased cared strongly for. So the traumatised soul ends up in a robot body wearing their beloved's skin and with no free will of their own. Nightmare Fuel much?

Comic Books

 * The Archaeologists in Requiem Chevalier Vampire are resurrected with no skin and have to spend most of their time floating in tanks; when one of them needs to venture outside their servants promptly flay some poor schmuck alive (with lots of screaming and thankfully some discretion shots) so the master can wear their skin.
 * In ROM Spaceknight, Dire Wraiths don't have to kill the people they replace, but they prefer to for obvious reasons. Their preferred method is to drill their tongue into the victim's brain to absorb his memories.

Film

 * The T-1000 from Terminator kills them and mimic their appearance.
 * Cromartie pulls a similar schtick in The Sarah Connor Chronicles; growing a new synthetic skin, undergoing plastic surgery to resemble an out-of-work actor, then killing him and assuming his identity.
 * Another terminator,  And
 * Also happened with the Terminatrix of the third movie. The current Arnie even lampshades it.
 * Terminator: Salvation has Skynet use Brain Uploading and the tech used to build the Arnies to resurrect Marcus. He's essentially a cloned and genetically modified human brain, heart and skin over a termie endoskeleton. So it's not that he's a cyborg with artificial limbs, but a robot with artificial flesh. Yikes.
 * In The Sarah Connor Chronicles episode "Allison From Palmdale" it is revealed that Cameron is a machine doppelganger of  Later on, after being captured by the human resistance, Cameron suffers damage to her processor that results in
 * If Invasion of the Body Snatchers fits anywhere, it's here.
 * The Edgar-Bug in the first Men in Black movie skinned and wore an "Edgar Suit" that rapidly decomposed throughout the movie.
 * Quirky Miniboss Cobalt Claw does this to an Office Lady Alpha Bitch in the Cutey Honey live-action movie. She then proceeds to wear the "suit" backwards while crawling on the ceiling, so apparently it was just For the Evulz.
 * It's implied in the Transformers films that the Decepticons made a point of destroying the vehicles they mimic, partially for infiltration reasons (Blackout caused a great deal of confusion by being a helicopter that was previously destroyed) and partially because they have no regard for human life. The Autobots are, unsurprisingly, more discrete about acquiring their vehicle modes.

Folklore and Mythology

 * Older Than Print: The Fair Folk were said to operate this way in English folklore, kidnapping babies and replacing them with lookalikes of their own kind known as "changelings".
 * Foxes do this too
 * In Korean mythology this trope is especially in effeect. Accounts vary, but typically the fox finds someone with the right skull dimensions, kills them, eats them, and then puts on their skull and assumes their identity. For different periods depending on the purpose; maybe just long enough to get into the house and eat your baby, maybe longer.
 * Japanese foxes are considerably less Exclusively Evil, and much less likely to need your skull to impersonate you, but their doing so can still have negative effects on the impersonatee. (Skull size is a factor for them in matters of possession, mostly.)

Literature

 * In The War Of The Dreaming, selkies refer to the skins as "jackets," and they can be made from any species' flesh. Weirdly enough, this is also played for comedy: high-ranking selkie switch skins so often the lower ranks are perpetually confused about their identities.
 * In John Dies at the End, Korrok's clones kill/replace the originals and proceed to go about their lives with all the memories of the original. For added authenticity, although the clones can be remote-controlled in emergencies, the replacements themselves lack alien memories and have no idea they're not the originals. This eventually leads
 * The premise of Impostor is that look-alike copies of key people can be sent after targets, exploding violently once contact is made. The hero is accused of being one such impostor.
 * Codex Alera's "watercrafting" can be used to imitate the appearances of others. This leads to a shock for one of our protagonists;
 * There is a reversal of this trope in Isaac Asimov's Evidence, where it is implied that a man who was crippled in an accident created a replicant for himself, who replaced him by his own consent,
 * Philip K. Dick's short story The Father-thing. When an alien takes the place of the protagonist's father, he eats his insides, leaving only a dry, dead skin behind.

Live-Action TV

 * The Slitheen from Doctor Who kill ministers, skin them and wear the 'suits' using gas compression technology.
 * Played much more tragically in Series 6, where
 * Captain Scarlet and the Mysterons' eponymous Mysterons fit this trope to a T. "Possessing the ability to re-create an exact likeness of an object or person -- but first, they must destroy."
 * In DS9 episode "Homefront", Sisko convinces the president that security measures are needed to prevent just that.
 * Happens from time to time in Kamen Rider Kabuto, as the Worms can shape-shift into a perfect copy of any human. The most famous example is, who we later learn is a Worm who Became The Mask.

Tabletop Games

 * The Tsochar in the Lords of Madness supplement for Dungeons and Dragons are tentacle monsters that can insert themselves into a humanoid host and either tag along harmlessly or violently usurp the original person.
 * There are also doppelgangers, changelings, demonic and ghostly possession, several magic spells... Inventive players or DMs can find dozens of ways for a character or monster to replace someone or wear him like a puppet, with varying degrees of survivability for the victim.
 * Exalted has this as an ability of the Lunars; they can become another creature if they ritualistically stalk it for hours, kill it, then drink its heart's blood. They can do the same thing for humans, but they can only pick up the specific form of the human they killed. However, there are Knacks that allow them to shift the appearance of a form they've acquired, and, if they're feeling humane, Knacks that allow them to assume a form temporarily after partaking of a non-lethal amount of blood from their target, or take a human form permanently after knocking them out, or even sleeping with them.
 * This is the central concept of 44, in which the PCs are people who've had a close relative or friend replaced by a robot, and the GM controls the Section 44 conspiracy. Brilliantly, player characters can be replaced during the game, and join the GM on the bad guy side.
 * The roleplaying game Changeling: The Lost plays on this idea. The changelings of the title are actually the humans who were abducted (at any age, not just as babies) and taken away to Faerie; fetches, artificial beings crafted from random detritus and animated by The Fair Folk, take their place, and actually believe themselves to be the person they replaced. Getting your old life back may very well involve doing this, in reverse, to an innocent being who's totally unaware that it isn't really you.

Video Games

 * The trope picture comes from the Sega CD game Snatcher, despite its oversimplification of the eponymous robots' preferred method of impersonating humans: grafting the replica skin and muscle tissue of their quarry over an exoskeleton in a People Jar.
 * The Soultaker from Limbo of the Lost stole 's skin to hide itself from the populace.

Web Comics

 * It was recently revealed in the webcomic Starfire Agency  when he's abducted by aliens (again) and has the situation explained to him.

Web Original

 * The Toymakers Workshop, being based on the Changeling: The Lost example, is about the construction of a fetch.

Western Animation

 * This is the purpose of the eponymous conspiracy in The Zeta Project. Its purpose is to allow the government to assassinate anyone it wants; they then send a holographic robot duplicate to prevent suspicion among the target's acquaintances.
 * Darkwing Duck had an episode with alien cabbages. The cabbages would sprout a clone, then the cabbage would devour and capture the original.

Real Life

 * Serial killer Ed Gein is known for wearing the skins of his victims, he wasn't trying to impersonate them though, just look like a woman.