In the Berserk series, demons used to be humans, and will revert to their former human selves after they die. This causes problems for Guts from time to time, but especially during the Rosine arc, where the Big Bad's pseudo-Elf minions were Apostle-spawns created from human children, leading the populace of the village she menaced to believe that Guts is a child murderer.
It's heavily implied that this is true for all Homunculi, but the only other one shown is Pride, whose true form is a tiny baby, about the size of a thumb.
There's a minor but straight "after death" version with Wrath who, while dying, suddenly looks like an actual sixty-year-old man, whereas previously, he looked a decade or more younger. The same is also done for Hohenheim.
Father after he is defeated and thrust into the Gate reverts to his true form, the tiny blob of darkness that Hohenheim once kept in a flask.
Bleach anime episode 291. When Kaname Tousen is stabbed and mortally wounded while in Hollow form, he returns to his true form just before he dies.
Captain America's body turns into that of a very old man after his death (though it is implied this takes several hours.)
The Hulk rarely (if ever) gets his ass kicked. Or at least takes a hit powerful enough to bring him down in one go. When he does however, occasionally it depicts him transforming back into Bruce Banner.
New Avengers: Someone they thought was Elektra is fatally injured...and turns into a Skrull. No-one had any idea she wasn't the real Elektra, including Wolverine and Doctor Strange, meaning that this faction of Skrulls has considerably improved their shapeshifting powers. Naturally, they start to wonder who else might be an imposter...
This also seems to happen to any part of a Skrull that's separated from the main body; when Crusader's hand was cut off during a training exercise in Avengers: The Initiative, it started turning a little green by the time they'd reattached it.
After the werewolf in the fifth issue of The Monster of Frankenstein is killed, it reverts back to human to reveal that it was in fact the woman whom the Monster had protected throughout the story.
Drake in Blade: Trinity actually managed to hold on to the last shape he shifted into for a few hours after death, having mimicked Blade as a parting gift to fool the authorities into thinking he was dead.
Although some fans speculate that Drake survived the fight and only changed back after waking up.
Applies in Underworld. Lycans normally revert to their humanoid form when they die, but Vampires eventually discovered a serum that would keep them in their wolf form after death so they could be studied.
The werewolves in An American Werewolf in London immediately revert to human form upon dying. The werewolf that attacked David and Jack turned out to be an old man.
This seems to happen to most supernatural foes the eponymous hero in Van Helsing faces, including Mr. Hyde and at least one wolfman. Van Helsing even remarks on this twice, telling his boss, "I'm the one who's left standing there when they die and become the men they once were!" He also says to the female lead, whose brother was a wolfman that Van Helsing killed, "Now you know why they call me murderer." He's even a wanted criminal since the authorities always catch him near corpses.
In the Species II the Half-Human Hybrid woman Eve turns back from her latex-monster form to human form after being knocked unconscious. Likewise in the Species III the hybrids always turn back to human form when they die.
The Matrix: Agents are capable of taking over bystanders' bodies. If they should be killed, the program leaves and the innocent most recent host is unmistakably dead. There are no other bodies, but all their other hosts are presumably very dead as well, or they just find themselves someplace strange with no memory of how they got there.
Averted to a degree in the Hammer HorrorThe Curse of the Werewolf (1961): when Leon the werewolf is shot by his father, the father covers his body with a cloak and the audience does not see if it reverts or not.
Played straight in both the original The Wolf Man (1941) and its remake. In fact, in the original, this causes some problems for Larry - he beat a wolf to death with a cane, and is very confused as to why everyone keeps asking him why he killed a human.
In the 1933 film version of the H. G. Wells novel The Invisible Man, the title character became visible when he died.
Fright Night (1985). After Evil Ed is killed while in wolf form, he returns to human form.
Mizuno, the eponymous character, reverts to his solid form after the theater is blown up in The Human Vapor.
The Simians, or aliens form the Third Planet of the Black Hole, in Godzilla vs. Mechagodzilla revert back to apes upon death (full body) or when hurt (only their injured parts). This is only true for this movie — in the sequel Terror Of Mechagodzilla, only one dies on-screen, but instead of warping into an ape, he rather arbitrarily tears the skin off his face, revealing a deformed head. It's a clear but kinda random reference to the Planet of the Apes movies.
In The Happy Ghost 5, a dog makes a deal with the Happy Ghost to become a human for 44 days because he wants to do more for his owner before dying. At the end, he takes a bullet for his mistress and is loaded onto an ambulance. Seconds later, the paramedic remarks that he can't do anything for a dog. The woman then sees her dead dog on the gurney.
The werewolf in The Undying Monster immediately turns back to human when its shot at the climax in the cliffside, and it falls to the crashing waves below.
In Red Riding Hood, the beast hunter they bring in tells them they haven't killed the werewolf because they have a wolf's head mounted, not a human head. He also relates the story (an actual werewolf myth) that he once tracked down a werewolf and cut off its paw. When he got home, his wife was cradling her bloody arm, and a look at the paw revealed it was now a human hand, wearing her ring and revealing her as the werewolf. She's still alive, so it isn't clear if the paw transformed because it was cut off or because it continued to transform as she did. At the end of the movie, when they actually manage to kill the werewolf, he does take his human form.
Established as a shapeshifter in Bounty Hunter, this happens to Zam, the bounty hunter who is chased by Anakin and Obi-Wan in the beginning of Star Wars: Attack Of The Clones, after she is poisoned by Jango Fett.
When the heroine of Cat People dies, she shapeshifts back into a panther.
Referenced and subverted in a The X-Files tie-in novel. Standing over the corpse of a woman with natural camoflage, Mulder laughs out loud when he realizes that he's been waiting for her to turn back to normal.
In the Michael Moorcock novel The Vanishing Tower, the shape shifting Oonai change back into their true pig-like forms when they die. Moonglum observes, "It's not hard to see why an ugly creature like this would wish to change its form."
The Katagarians revert to an animal form, and the Arcadians revert to a human form when they are unconscious or dead in Sherrilyn Kenyon's The Dark Hunters series.
In Eddings' The Belgariad/The Mallorean the Hounds of Torak do this. They are Grolim priests—minor sorcerers—transformed into enormous dogs, but they go back to being human if you kill them.
In The Lando Calrissian Adventures, the Big Bad is basically indestructible throughout the story. Turns out his species is actually quite small and non-humanoid, and is using illusion powers to simulate the form we know him as throughout the stories. When Lando scores a lucky shot at one foot, he strikes the real Gepta for the first time, and with the illusion gone, he's able to finish him with one squeeze. It's a bit more justified then most, in that Gepta makes it clear his illusion powers are ACTIVE illusion.
The Picture of Dorian Gray: When Dorian Gray stabs his portrait, he ages to his "supposed" age and dies, while the portrait reverts to depicting his young form.
In Welkin Weasels: Vampire Voles, the "wereweasel" shot by Monty changes back from a monstrously mutated six-inch-tall human into a normal weasel upon dying.
Shapeshifters in the Kate Daniels universe assume their birth form when dead or dying. Results in an "oh, crap!" moment when Corwin's in the hospital and suddenly transforms into a lynx. He dies soon after.
In Robert R. McCammon's The Wolf's Hour, the body of a werewolf spasms at death, becoming a mishmash of wolf and human parts.
Non-death example: In Harry Potter, Metamorphmagi (wizards who can change their form at will) apparently lose their power under extreme depression. Happens to Tonks in Half-Blood Prince while she's pining over Lupin.
Also, the "raw"-looking baby-like form that is Voldemort with only a fraction of his soul left gets transformed back into a relatively normal-looking adult human by Wormtail's spell in book four, but it's the baby version Harry sees in his Afterlife Antechamber, while he and Dumbledore look like their ordinary selves.
Averted in Kelley Armstrong's Otherworld books, a werewolf remains in whatever shape it was in when killed.
Treasure in the Heart of the Tanglewoodoutright subverts this trope towards the end. The villain initially claims that each Knight in Shining Armor was an animal turned into a human, making it a simple matter for him to turn them back into animals and kill them. He lied—while each had been a victim of a since-cured case of Baleful Polymorph, reapplied in the moments before their deaths, their initial forms were human. They still stayed animals when dead.
Subverted in Heart of Midnight, a Ravenloft novel, where protagonist Casimir is a wolfwere rather than a werewolf. Harkon Lukas, his biological father, expects Casimir to revert to wolf form when he dies, but because he's struggled his entire life to be human rather than a monster, he remains that way in death.
The goblins in Twilight Eyes by Dean Koontz were genetically engineered to switch between human and monster form at will, and to revert to human form after death.
The main character, Horza, in the Ian M. Banks novel "Consider Phlebas" reverts to his true form after his death (but not when unconscious, that would be terrible for a member of a race of shape shifting spies and infiltrators). Considering his borderline Shape Shifter Identity Crisis earlier on, it's not clear if this dead form is actually the face of the real Horza (assuming such a person even existed) or if he had reverted to whoever he was originally underneath all the infiltration training and psychological conditioning. The whole thing is presented as a Bitter Sweet Ending as the closest thing he has to a friend looks down on his body and wonders who he really was in the end..
At the end of the Mistborn trilogy, Ruin, the god of destruction, is killed, and he leaves behind- a human corpse, leaking the black smoke that had been previously identified as Ruin's divine power. Word of God is that Ruin was actually a combination of a human intelligence named Ati and the cosmic force of entropy- his death knocked the two apart, leaving the human body of Ati behind. The same had previously happened to his opposite, Preservation, though none of the characters witnessed it.
Conan, who had not shrunk from Khosatral living, recoiled blenching for Khosatral dead, for he had witnessed an awful transmutation; in his dying throes Khosatral Khel had become again the thing that had crawled up from the Abyss millennia gone.
Averted in Who Goes There??, the short story that inspired The Thing (1982), the alien starts off as a blue humanoid with three malevolent red eyes, blue skin, and tentacles on its scalp. Then it eats someone and mimics him. It's strongly implied that the initial humanoid shape is no more its "true form" than the human and dog shapes it takes later, because Things don't really have a true form ... they're more like colonies of microorganisms that take on whatever shape seems useful.
In The Last Dove, anyone who can Change tends to sleep in the form that fits their true self more. Considering the fact that no major characters die in the book, it's impossible to know if they would do the same when they die.
In Andrei Belyanin's The Plot of the Black Mass, a villain uses a spell to turn temporarily turn himeself into a fly in order to kill a suspect in jail. However, the guy left in charge is a simple-minded farmboy with enormous physical strength. He proceeds to swat the "fly" and smashes it against the wall. Cue the horrifying scream from the farmboy when the villain turns back into his human (albeit squished) form.
The Doctor Who episode "The Lazarus Experiment" did it with a twist: Elderly Professor Lazarus is restored to youth by an experimental technique that then goes wrong and turns him into a hideous monster. When the monster is apparently killed, it reverts to Lazarus' human form; when it's really killed, it reverts to Lazarus' original elderly form.
In "Survival", the last episode of the classic series, Karra, a werecat cheetah woman, reverts to human form as she's dying.
Averted in Buffy the Vampire Slayer, where werewolves remain in werewolf form after dying. A werewolf hunter out for pelts even comments that "it's a little hard to skin them while they're alive."
And then used straight in Angel, as it's said werewolves revert back to human form upon death and thus must be eaten alive. A possible explanation is that werewolves of the species Lycanthropus Exterus change back to human upon death, whereas the "common" werewolf does not.
In another episode, a man has turned himself into a monstrous creature as part of a plan to rescue his half-demon son from its mother's species. He specifically mentions that he would only turn back when he dies. However, he's later fatally injured and turns back seconds before he would die, giving them time to heal him. He remained a human, though.
Tweaked in Sliders. A vanquished fire-breathing dragon reverts to its (true) human wizard form as it lays dying...and then becomes an even smaller cockroach when no one is looking, allowing it to scamper away. Only to get stepped on moments later.
Tin Man: It's not quite clear what to make of the fact that Tutor changed back into Toto when he died... aside from the fact that a dead puppy is a somewhat more tearjerking visual than a dead middle-aged heavyset guy.
In Heroes, the illusionist Candice is knocked unconscious, but reverts to her hot and skinny illusion form. When she's actually killed, she reverts to her true form, which is an obese woman.
Completely averted with the death of shapeshifter James Martin, who retains the appearance of Sylar, causing everyone except Sylar and Emile Danko to believe that Sylar is dead.
Used in Farscape when Jack the Ancient dies, reverting to his true insectoid appearance.
Also in Farscape when Nam Tar is injected with the genetic reversal serum he reverts back to his original primitive ratlike form.
In the Star Trek epsiode "Catspaw", the two dying villains are revealed to be spindly blue aliens.
"The Man Trap", has a shape-shifting alien that avoids capture by mimicking different members of the ship's crew. It finally returns to its true (hideous) form after Spock is forced to put it down to save Kirk.
Star Trek: Deep Space Nine: Changelings must revert to their true form, a liquid, every day to rest. They also revert to this liquid if they are killed.
Misfits, when Kelly's new boyfriend is shot, it's revealed that before the storm, he was a gorilla.
Occasionally inverted with HAPs in Sanctuary. Even if they die in human form, in death their bodies will sometimes show signs of their werewolf nature. For example Henry in "Pavor Nocturnus" or Joe in "Animus". Alistair on the other hand remains completely human after dying.
Happens to werewolves and shapeshifters in True Blood upon their death: they instantly revert to their naked human form, the injuries they sustained in animal form still carried over to their real body. At one point, Bill the vampire is trying to kill a shapeshifter that escaped him by changing into a fly. He tells his guards to crush every bug in the building to see if it shifts back. Fridge Horror to what exactly a crushed bug shapeshifter would turn back.
A minor version occurs with Elemental Shapeshifter Carl Creel in Agents Of Shield. He's bulletproof during his first encounter with Team Coulson, but they do recover a piece of shrapnel from his then-metallic body after the fight. It reverts back to flesh and blood while Fitz is analyzing it, which makes the team realize that they're dealing with a Gifted.
Religion and Mythology
Generally Inverted with the Youkai: when they are killed, sufficiently battered, or simply running out of magical fuel, they revert to their non-human form. Fox youkai will revert to being a fox, cat youkai will return to being a cat, etc.
Various cultures in the world play this straight. In an example, some people found a stray goat or caught a wild boar. They slaughtered it, cleaned the content of its stomach, and then left the carcass for a while. When they return, they found the dead body of a man in their kitchen, missing his intestines.
A variant has someone cut off a part of an animal (usually the paw of an animal like a wolf, cat, or rabbit, though sometimes it's the tail of a snake), only for it to change back into a severed hand/foot. This usually results in the person noticing that the limb looks very similar to those of a loved one...
Werewolves in Werewolf: The Forsaken turn back into humans when slain, which helps them keep the Masquerade up. This also applies to any lost blood or body parts - as the book puts it, "a werewolf can spill a gallon of blood while in the war form and it will all register as human".
In its predecessor Werewolf: The Apocalypse, the Garou normally revert to their base form when rendered unconscious, as would their body parts if mutilated. The problem is that some are born in their 9-foot-tall war form and must shapeshift into normal humans or wolves. Sure enough, they revert back to the war form whenever they fall asleep. An expensive Merit gives player characters exceptional control over shapeshifting, including the form they take when knocked out. Storytellers are prone to disallowing it, however.
Any kind of shapeshifting magic (even if it's a god's inherent ability) results in this.
Ravenloft module RM3 Web of Illusion. If PCs kill a weretiger they encounter, it will change back into its human form.
The Basic D&D Master Rules "Dungeon Masters' Book" said that lycanthropes (e.g. werewolves) return to normal (human) form when killed.
FROA1 Ninja Wars. Takako Shimizu was infected by an unusual tigbanua biso and becomes a tagamaling buso at midnight each night. If she is killed while in tagamaling buso form she will change back to her human form as she dies.
Champions adventure The Coriolis Effect. The Black Enchantress changes a number of people into slavering monsters. If the PC heroes kill them, they change back into their human forms - still dead. Nice Job Breaking It Heroes.
Shadows of Yog-Sothoth adventure "The Warren". Philip Boucher is possessed by and changes into the Cthulhu Mythos deity Y'Golonac, then tries to kill the PCs. If they manage to do enough damage to defeat Y'Golonac, Boucher's body will return to normal, but he will be dead.
Masks of Nyarlathotep chapter 2 "London", adventure "The Derbyshire Monster". If the werewolf Eloise Vane is killed while in monster form, she makes a final transformation to "ground state" (human form).
Double subverted with a murder victim in The Wolf Among Us. While Bigby appears to find the head of Snow White on his front steps, it's not her; it's someone else's head, under the effects of a glamour. While the victim is killed without changing back, the double subversion comes in when the glamour's magic eventually wears off, where the victim reverts to her true form of a cave troll.
Also in Final Fantasy VII, if Vincent is KO'ed during his Limit Break (where he shapeshifts into various monster forms), he'll shift back to his human form shortly after collapsing.
Averted in one case in Final Fantasy VIII, it is implied that the Shumi tribe have multiple forms, some terrifying and vicious. NORG, a member of this race, is defeated early on in the game and his location remains a visitable point throughout the game. While the player might expect NORG to return in another "true" form to take his revenge, he never does.
In Diablo, when you slay Diablo and pull the soulstone from his forehead, his body reverts to that of Prince Albrecht, whom Diablo had possessed. (It is unclear whether Albrecht is alive or dead at this point.)
Oddly enough, in Diablo II, there's no sign of this when the player removes Mephisto's soulstone from the body that used to be Sankekur. Possibly due to Diablo's "death" in the first game being planned while Mephisto's death wasn't
Roguelike Example: In NetHack, the player (and monsters) can polymorph through various means. A polymorphed creature will return to their true form if their HP drops to 0, but strangely, this restores their HP to full. Dying in other ways, like from hunger or petrification, still kill you without changing your form back.
Averted in Knights of the Old Republic. A side mission has you hunt down and kill a shape-shifter, and he goes between a Wookie, one of your party members, a giant monster, and a small ape (he was trying to beat a hasty retreat), and after you finally kill him he reverts to a charred and unidentifiable skeleton. We shall never know his true form...
Technically, it is possible that that skeleton was the skeleton of the shapeshifter's true form, given that it evidently is not the skeleton of the form that was killed.
This behaviour is also utilised with one of his alternate watches, that causes him to appear to "die" if he is hit, but instead simply drops a replica of his corpse and turns the player invisible, allowing the player the chance to then de-cloak and continue on their way.
Unless he's disguised as another member of his own team, then he leaves a fake corpse of one of them.
Druids that die in animal form instantly revert back to their default form in World of Warcraft. This also goes for characters killed while polymorphed.
Averted with Worgen, however, whose corpses will stay in their human forms if they manage to die without entering combat. They even appear at the graveyard as human ghosts.
Subverted and lampshaded in The Elder Scrolls series. Slain werewolves retain whichever form they were in upon their death, and never change back. This was mentioned in the book "On Lycanthropy" in Daggerfall.
Von Zell, a werewolf, reverts back to human form after being killed in the second Gabriel Knight game.
In Mega Man X8, when you defeat each Maverick in the requisite rematch stage, you'll find that they are really next-gen Reploids who shifted into the Mavericks' forms, and they revert to their true forms before they blow up. Taken to its logical extreme when even your first "Sigma battle" turns out to be a shapeshifter, and every minor enemy in the final stage is a next-gen Reploid imitating Sigma. They all shift back to their true form before they are destroyed.
The Fake Bowsers from Super Mario Bros.. They are actually seven enemies transformed by Bowser into exact copies of him to distract the Mario Brothers throught the first seven worlds while Bowser holds Peach captive in the eighth. The only way for the Fake Bowsers to reveal their true selves is to have either Mario Brother kill him with fireballs. Here are the Fake Bowsers encountered in the game:
Super Mario Bros.: The Lost Levels featured six new Fake Bowsers, two of them being found about halfway through worlds 8-4 and D-4. These Fake Bowsers, however are slightly bluish in appearance (in Super Mario All-Stars, they are colored normally), and are optional. The new Fake Bowsers encountered are:
A fairly accurate comparison here, but these fake Bowsers may be likened to King Boo's fake Bowser in Luigi's Mansion.
In the story mode of Mortal Kombat vs. DC Universe, when Liu Kang first meets Flash, he thinks that he is Shang Tsung in disguise (he had been magically forced to switch places with Scorpion for some reason), but he decides otherwise after defeating him, saying that "if this was Shang Tsung, he would have changed back by now."
Immortal Souls plays this straight with its werewolves changing back into (naked) humans when killed. Though oddly it only happens to members of a werewolf gang and not werewolves found elsewhere.
Hyperdimension Neptunia does this in a strange way - if any of the CPUs on your team (Neptune, Black Heart, White Heart, or Green Heart) lose all their HP in battle, they revert to their normal form just before collapsing. When fought as bosses, however, they simply disappear like any other enemy.
In the third Phoenix Wright: Ace Attorney game, the last trial centers around the death of a children's book author. It turns out that the author was Misty Fey, mother of Mia and Maya, and that to try to foil a plan to kill Maya, she channeled the spirit of and shapeshifted into her murderous niece, Dahlia Hawthorne. Dahlia still set off to kill Maya, so Godot stabbed Dahlia, resulting in Dahlia being forced back to the afterlife and the now-dead Misty changed back to her own appearance. This leaves Godot with the issue of how to dispose of Misty's body.
Dragon's DogmaDark Arisen: When Daimon is defeated, his body begins to melt into the ground. For a moment, the shape of his original body as a former Arisen can be seen...before that too melts away.
Sev Trek: Pus in Boots (an Australian CGI spoof of Star Trek: The Next Generation). The crew find themselves confronted with two Captain Pinchhards, one of them a shapeshifting alien. The crew wonder how they can Spot the Imposter. Lieutenant Barf responds, "Simple, we kill them both. The alien will revert to its natural form once dead." Needless to say the captains tell him to think up another plan.
In Star Wars: The Clone Wars, as Savage Opress is dying, he reverts back to his original appearance, looking as he did before the Nightsisters transformed him.
Non-death example: In an episode of Futurama, Leela meets another cyclops, whom she almost marries. At their wedding though, it's revealed that he had another four wedding to attend that day with other nearly extinct aliens. Leela and the other four brides beat the crap out of him, demanding to see his true form, until eventually he submits.