The manga of Chrono Crusade has an unusual example revealed towards the end: Pandaemonium was once a human woman pregnant with twins. The demons kidnapped her and transformed her into the monstrous, Mind Rape-using queen she is — and transformed her human children, Chrono and Aion, into demons as well.
In Xam'd: Lost Memories, it becomes clear fairly early on that both humanform weapons and Hiruko are former humans.
Each and every of the Awakened Beings in Claymore. And, perhaps more shockingly, the Abyss Eaters. Even more shockingly, the Youma.
Anna from Elfen Lied... Dear God, Anna. When first introduced, she is an adorable and happy, if dimwitted, young girl who loves to run. Her father is disappointed with her lack of intelligence, and transforms her into an enormous, horribly mutated creature with super-intelligence and precognitive ability. She cannot support the weight of her gigantic head, and so must remain virtually immobile in a pool. Remember how she loved to run? Yeah.
King of Thorn has the "mother monster", which is eventually revealed to have been a human (Shizuku) who succumbed to The Virus.
World Embryo - The enemy virus, Kanshu, were once humans who lost their memories and transformed into hideous beings upon listening to the infected radio signals in their cell phones.
Bleach. Every Hollow started out as a lost human soul. Eventually, it lost its heart, whether through time or the attacks of another hollow, and became a monster, feeling nothing but the desire to murder and a hunger for souls.
If they survive long enough, Hollows can "get over it" on their own (or if someone else removes their mask or uses the Hogyoku on them) and become Arrancar, who are intelligent creatures and apparently don't need to eat souls anymore (considering that they have copious amounts of regular food in Las Noches and only three or so Arrancar are ever shown any desire to eat anyone... and of those, one is a Mad Scientist who's turned his minions into edible medicine and another has the ability to gain the powers of anyone he eats, so they're not just eating for the sake of eating.)
Similarly, the Akuma of D.Gray-Man. As with Hollows, their hunters know, at least in abstract that they contain the souls of innocent people forever tortured until the Akuma are destroyed and they're released, but only Allen has to confront the vision of them on a daily basis. As with Hollows (though in Bleach it's a different story), leveling up obscures more and more the original souls, until even Allen can't see them anymore. It also makes new souls in the process.
Some of the monsters in Sailor Moon, were (for the most part) originally human. Some of the Youma in the first season and the Pharge of the last season. Sailor Moon's power to restore the Pharge to normal actually shocks the Starlights, as they had to simply destroy them because they lacked the ability to do so. (Though they implied their Princess could, as they stated they had to destroy them without her around). A single human-based Daimon also appeared in a flashback which was closer to their manga version.
In Fullmetal Alchemist, Shou Tucker, the Sewing Life alchemist, fuses his adorable daughter Nina with her dog, Alexander. He also fused his wife with another beast a few years back, in order to gain his certification. Both chimeras are able to talk, and they both made it known they were not happy with their new existence. The one made of his wife asked to be killed and when it wasn't obliged it starved itself. The one made of Nina and Alexander is killed by Scar.
The dragon Acnologia from Fairy Tail was originally a slayer. After killing countless dragons he was apparently a turned into one himself by Zeref.
Yubel, the Big Bad of season three of Yu-Gi-Oh! GX. She willingly gave up her humanity and was turned into a demon in order to become a spiritual guardian for her ward (who would become Judai in a later life).
All of the villains originating from the Bad Future of Yu-Gi-Oh! 5Ds, including Z-one, Aporia, and Paradox. The first is a cyborg with god-like powers, while the other two are android incarnations of their previous selves.
The Seven Barian Emperors, the antagonists of the second season of Yu-Gi-Oh! ZEXAL, were all once human, and they can assume human forms that resemble - for the most part - their original forms (at least that is true with the ones whose original forms have been seen in flashbacks; we can only assume that is true with Vector, Gilag, and a couple who have yet to take human form).
One Piece has the island of Dressrosa. There humans and living toys live together. Except unlike other fantastic beings in One Piece, these "toys" were once humans but turned into toy creatures by a Devil Fruit user.
In Attack on Titan, Titans were once human. The only difference between Shifters like Eren and regular Titans is that he was able to control the power. Regular Titans appear to have fully absorbed their human "pilot", reducing them to mindless horrors that seek out humans to eat. Ymir was able to regain her humanity after 60 years, though the exact method is unknown. It is, however, hinted to involve the fact that she ate a suspected Shifter around the same time....
Invoked in Strikeforce: Morituri with the "mutants", four humans who were accidentally turned into super-powered monstrosities when they underwent the Morituri Process without proper supervision.
Galactus of the Marvel Universe was once a man of the previous universe named Galan. As his universe died to pave way for the new, current one, Galan journeyed into the center of the Big Bang refusing to give in to destruction. The powers that be were impressed and transformed him into Galactus.
In B.P.R.D., it is revealed that Abe Sapien was once a human named Langdon Everett Caul, and has been alive since the 19th century.
Of course, there are several other examples in both BPRD and Hellboy, such as the frog monsters, or the Too Dumb to Live wannabe satanist aristocrat couple who fell afoul of minor demon Ualac in Box Full of Evil.
Played with in Paperinik New Adventures: the surviving Xerbians initially consider Xadhoom an alien monster (albeit an allied one), and are quite unsettled when they find out she was once their leader. Them expressing the wish to make her return to normality also caused them to receive a "The Reason You Suck" Speech (with A God Am I thrown in to hammer down she has no reason to return normal) before Xadhoom went for her Heroic Sacrifice.
Solomon Grundy was once Cyrus Gold, a merchant in 18th century Gotham City, who was murdered and whose body was dumped in the infamous Murder Swamps, where he rose once again as the unkillable Grundy, who would walk the Earth forevermore.
Man-Thing was a scientist who, upon his death in a Florida swamp, was merged with an attempted recreation of the Super Soldier serum that created Captain America that he had been working on, and became a hideous half man, half plant creature with only the most basic remnants of sentience left.
Swamp Thing was once Alec Holland, who mutated into a swamp creature through similar means as Man-Thing above. Or so he thought. Alec Holland died out there in the swamps, and the being that rose from the mud was a plant elemental with the absorbed memories and personality of Alec.
Salem from Sabrina the Teenage Witch was changed from a normal cat to, in the late 90s, into once having been a wizard who was punished to live 100 years as a cat because he tried to take over the world.
In the Portal2 fanfic Blue Sky this is revealed to be true of Wheatley, though he isn't pleased to realise it. He eventually makes it back to his real body.
A lot of human!Wheatley fanfics invoke this.
In the My Little Pony: Friendship Is Magic fanfiction Legends of Equestria Discord reveals that he was once a unicorn in ancient times. However, he was transformed by the corrupting power of dark magic after accepting it in an attempt to restore order to his life after his family is killed.
In Plasma's Folly, Kelvin reveals that Genesect is actually a middle-aged woman who had become terribly deformed as a result of Team Plasma's attempt to create a Pokémon with human intelligence. She was stuffed into a suit of robotic armor, which is the only thing holding her body together and keeping her alive.
In Rise Of The Guardians, while all the Guardians are more supernatural beings than monsters, Tooth implies that they were all mortal before becoming who they are now. Jack was a human who had drowned before the Moon transformed him into an immortal being.
Films — Live-Action
In the horror film The Cave, one of the monsters bears the same tattoo on its hand as one of the cavers previously seen entombed within the cave system at the beginning of the movie. Which bodes poorly for parasite-infected expedition leader Jack.
The Reavers in Serenity. Actually, it was BECAUSE their minds were warped that their bodies are so messed up (self mutilation).
The Cenobites in Hellraiser films. (And when a Cenobite is killed, his or her body turns back into human form, suggesting that killing one of them may be a Mercy Kill in some cases.)
In the 1973 thriller b-movie Sssssss, mad scientist Dr. Carl Stoner (Strother Martin) turns his lab assistants into king cobras.
Darth Vader probably qualifies, certainly in spirit: He's more machine than man now, twisted and evil.
General Grievous certainly does... well, he was Once A Kaleesh, but you get the idea.
The lizard-monster in The Relic is revealed at the end to be a human explorer who ate a concoction of some particularly funky herbs in South America.
Davy Jones in the second and third installments of Pirates of the Caribbean. Lampshaded by Calypso, who describes Jones in this manner (and is responsible for the curse that transformed him). His crew of mutated humanoid sea creatures also qualify.
The mutants in the Doom movie. Condemned criminals who were injected with an extra pair of chromosomes, but due to their DNA having "genetic markers for evil", horrifically mutated. And they can sense who else has the same evil genes, so they attack them as well.
The quote from the top of the page (now on the quotes page) is from The Beast in the Cave by H. P. Lovecraft, written at some point around 1908. Similar themes can be found in The Rats in the Walls, Facts Concerning the Late Arthur Jermyn and his Family, and The Shadow Over Innsmouth.
Pech in The Dark Elf Trilogy technically was never human, but he was a harmless sentient creature turned into a monstrous hook horror by a wizard.
J. R. R. Tolkien's Middle-Earth (The Lord of the Rings, etc): As individuals, there are the Nazgűl (once human rulers) and Gollum (once just an ordinary proto-hobbit). In Tolkien's concept, evil and the various Big Bads cannot create, only pervert: therefore, almost every evil creature (e.g. those used as mooks) is a corruption or mockery of a pre-existing being. The orcs are descendants of elves twisted by Sauron's predecessor, Morgoth. If you really want a squicky thought, it's rumoured that the Uruk-hai were partly Man in some fashion. Trolls are a mockery of Ents. Subverted in the case of the undead Barrow-wights, who only possessed the dead bodies of those buried.
The orcs-from-elves theory is only one possible theory - Tolkien never exactly clarifies where the orcs came from. Some dislike the idea that orcs were literally corrupted elves as that would imply the orcs have genuine sentience and intelligence, which Tolkien implies on a few occasions is not the case, so some suggest the orcs are corruptions of various non-sentient animals.
While Sauron was never a man as such, he was a Maia, who would have originally looked very much like an elf or man (he's specifically stated to have been quite handsome before the fall of Númenor, when his nice body was destroyed and he could no longer take a fair form).
The balrogs are also corrupt Maiar. Gandalf is strongly implied to be an uncorrupt example.
In The Relic, the museum beast is revealed to be a scientist mutated by an ancient retrovirus.
In Lee Lightner's Sons of Fenris when the Space Wolves and Dark Angels go up against Cadmus's elite forces, the tattered remnants of their uniforms is the only evidence they had once been human; some still wield weapons, but only those merged with their flesh.
Gav Thorpe's Warhammer 40,000 novel The Path of the Warrior reveals that Eldar Exarchs were once Eldar, but lost themselves in the struggle to control their rage and became part of a gestalt consciousness dominated by the first Exarch to lead their shrine, trapped and unable to die, subsumed into the whole, and speaking only in stream-of-consciousness.
In Star Wars: New Jedi Order, the mindless warbeasts known as the Vagh Rodiek were once Rodians. They were created when the planet fell to the Yuuzhan Vong.
Abeloth. She was once a mortal woman (species unclear) who served The Father, The Son, and The Daughter. She was eventually promoted to The Mother and loved her new family. As she aged, she grew paranoid that her ageless family would abandon her, so she drank of the Font of Power and bathed in the Pool of Knowledge. While this granted her immortality and advanced Force powers, this also mutated her into an Eldritch Abomination. Sadly, this event caused her family to abandon her.
In Neal Shusterman's Everlost, the monster called the McGill is revealed to have been Mary's brother, who sank down to the center of the earth and clawed his way back up. When he returned, he was a monster.
The Steel Inquisitors of Mistborn are humans who have been transformed into nigh-immortal killing machines via the dark art of Hemalurgy. The Koloss from the same series were originally humans as well, created by a similar process.
Also The First Generation of kandra were ALSO former human Feruchemists who were friend of the Lord Ruler before his ascension, the other kandra are descended from mistwraiths that were ALSO Feruchemists, who weren't friends of the Lord Ruler and so didn't get to be sentient after the Lord Ruler was done with them. Kandra are made from mistwraiths using the same Black Magic that makes Inquisitors and koloss.
Unusually for these tropes, it's not all that horrifying or morally dark. Kandra are intelligent and have no memories of being human, except for those of the first generation, so it's not traumatic for them. The Black Magic that turns mistwraiths into kandra is "black" because it is Blood Magic, sort of, and turns them into The Mole for the Bigger Bad, but that part doesn't come up until the end of the last book of the trilogy.
In Death Masks of The Dresden Files, Harry gets a nice shock to the system when he looks into the eyes of the latest monstrosity to cross his path and sees the human soul it has.
In Cold Days at the end, Queen Mab tells Harry that she was once a mortal. And Molly fills this as she becomes a Fae Queen as well.
And then there are also the three types of vampires: those of the Black and Red Courts are created in the typical way (humans turned through blood-drinking/exchange), while the White Court are born as humans (from other White Court) then turn into vampires sometime during their late teens/early adulthood.
Played with in Discworld. Unseen Academicals marks the first appearance of orcs in the series. During the course of the book it is revealed that they are a manufactured species made from goblins. Only as it turns out, that's a misconception. As Vetinari puts it, "Goblins wouldn't have been nearly as ferocious." Discworld orcs were made from men.
Also, the librarian (due to a massive magical event). He's perfectly happy being an orangutan for the rest of his life; just don't call him a monkey.
A brief part of Garth Nix's Old Kingdom series followed some sort of undead/free magic creature that used to be human.
Most of the various types of Dead were originally human in body and/or spirit (though some of the weaker ones, like the gorecrows, were once animals). Hedge starts out as a human necromancer, but becomes progressively inhuman as the Destroyer's power over him increases (thankfully, we never learn what exactly he was turning into, though it doesn't seem to have been Dead).
God Emperor of Dune Leto II (mostly internally or to those close to him) laments his loss of humanity after becoming a giant sandworm, but he knows this is necessary for the survival of the human race.
Used in the Angel novel "Image", which has a guy who once was human but became more and more demon (and grotesque) in order to stay alive for hundreds of years.
Subverted in The Island Of Dr Moreau: the narrator thinks that Moreau's creations are transformed humans, but they are actually animals that Moreau has turned into Beast Men.
Dr Franklins Island, inspired in part by the above entry, has a couple of teenaged girls transformed into mutant animals. They grow used to their new forms before too long, but are always aware of what they look like to others. One of the scientists, face to face with one for the first time since she woke up like this, is struck with horror and hopes out loud that she doesn't have the same mind.
Several of the monsters on Supernatural, the most important being the demons.
One later iteration of the Daleks in Doctor Who were produced by "filleting, sifting, and pulping" living humans to render a handful of cells judged strong enough to be shaped into Dalek form and welded into a travel machine.
Another Dalek faction, the Imperials, were made from Human Popsicles. "Not pure enough in their blobbiness" indeed. And the original Daleks were once the humanoid Kaleds.
The Toclafane are Axe Crazy flying metal spheres that are able to deploy knives and laserguns. Turns out that they once were humans living at the time of the universe's end. They turned themselves into metal spheres in hopes of surviving the end of the universe.
And of course the Cybermen, more than any of the others. It's arguably the whole conception behind the way they were originally written in the 1960s.
In the revived series, the Cybermen all look identical and have the same voice, so it's particularly jarring when Pete Tyler is confronted by a Cyberman that used to be his wife.
Many stories involving The Virus: e.g., "Mission to the Unknown", "Inferno", and "The Seeds of Doom".
Poor little Jaimie from "The Empty Child" / "The Doctor Dances".
The Face of Boe may have once been Jack Harkness, or maybe Jack was just messing with the Doctor, and Martha, when he said that.
Oswin in "Asylum Of The Daleks". The Daleks turned her into one of them, and she was unable to deal with it, instead dreaming up that she was still human.
In Power Rangers Time Force, Frax, the robot who worked for Ransik but would eventually go solo, was once a human named Dr. Fericks who saved Ransik's life in the past, but was rewarded with the destruction of his lab and body. After using his own technology to rebuild himself, Frax vowed revenge on Ransik, and infiltrated his organization to bring him down from within.
Master Org in Power Rangers Wild Force was a Doctor before taking on the powers and identity of the original Master Org. His minions weren't happy when they found out, but he proved to be too much for them when they tried to rebel.
Zeltrax was transformed into a cyborg after a lab explosion. He is not happy about the loss of his body, and has decided that it is (in a roundabout manner) Tommy's fault. Mesogog was once a human scientist too.
In Power Rangers Samurai, Deker and Dayu were once human. Dayu sold her soul to save the life of the then-human Deker, but Deker has lost his memory and is now a Blood Knight who fights to satisfy his bloodlust, either by defeating a Worthy Opponent or by being put out of his misery. Last time he fought the Red Ranger, it looks as if the latter has finally happened. However, the season's only half over...
All Borg in Star Trek started as other species, usually humanoid.
And Seven Of Nine is an actual human from the Federation.
The Man in Black on LOST claims to have once been a human before becoming a sentient cloud of smoke. He's now human again, only able to switch between his monster form and John Locke.
The eponymous character in the Black Sabbath song "Iron Man" was a well-intentioned human given metallic form by a "great magnetic field" while traveling time to save the future of humanity. He went on to go cuckoo and decimate the human race because after he saves them, they won't help him or accept him.
Myth And Legend
Most undead creatures are, naturally, like this. For example: zombies and vampires.
Quite a lot of animals in Greek myth were actually humans who ticked off the gods and were transformed into beasts - Arachne and King Lykans being the two most prominent.
Scylla and Charybdis also fall into this category.
For a full list, read Ovid's Metamorphoses.
Oddly subverted in the case of Heracles though. Born a man, his heroism leads him to be elevated to a god. Admittedly, one of his parents was a god, but there are plenty of other heroes in Greek mythology who have one divine parent, and Heracles is the only one who manages to become a god himself (though there are several humans who upon their deaths are made into constellations, trees, islands, stars, etc so that they will never 'die').
Fafnir, aka the dragon from Wagner's Ring cycle, aka the inspiration for Smaug in The Hobbit, Was Once a Man! In fact, he got the gold hoard first, and it cursed him for his greed.
His siblings, fyi, were a dwarf and an otter. No reason given.
His brother was a shapeshifter, as was he and his father, hence the otter thing.
The Bohrok in BIONICLEwere Once Av-Matoran. goes into Nightmare Fuel territory when you realize that Nuparu made the Boxor machines out of Bohrok parts, so the Boxors are made out of dead Av-Matoran... and are piloted by Matoran who use them to fight still functioning Bohrok. While he didn't pilot Boxors, one of the topmost fighters in the war against the Bohrok was Takua, himself an Av-Matoran!
A large number of Dungeons & Dragons monsters qualify, such as vargouilles, illithids (in which a larva eats your brain and uses your body as the foundation for its own), and skum (and pretty much all humanoid undead). A number of prestige classes, such as the alienist and fleshwarper, eventually become something inhuman as well (and not in a good way, like a monk's ascension to outsider status), although without necessarily being evil.
In Mortasheen, this is the case with the Arthropoids, a class of creature made from humans fused with various type of insects ala The Fly. This also applies to several of the player races, like the Borg, the Sectillians and the Zombies.
Chaos Spawn define this trope in both Warhammer and Warhammer 40,000. The Chaos Gods reward their champions with many gifts, including physical mutations. Champions rewarded with too many ill-considered gifts will eventually lose their sanity and become little more than beasts, herded into battle by their former followers.
Special mention goes to Scylla Anfinngrim, a champion of Khorne who devolved into a giant, hairy, shambling clawed horror. Unlike most spawn, he's proven nearly impossible to kill, and his tribe keeps him fed and worship him as the mighty warrior he was in exchange for getting to take a pure killing machine to war with them.
Fabius Bile is shown pulling this on a large number of Space Marines in the Horus Heresy novels. Only one or two retain some fragments of sanity after the mad Apothecary finishes tinkering; the rest are little more than rabid killing machines.
The Necrontyr were once a sentient organic species, but long ago had their consciousnesses uploaded into robotic bodies, becoming the Necrons. Among other things, this cost them their souls, which were devoured by their gods. Some Necrons have further afflictions: the relatively simplistic method that was used to transfer most of the civilians into their bodies left them as little more than automatons, while the Flayed Ones have been driven mad by the loss of their mortal bodies, and seek to recapture the sensations of life by garbing themselves in the flesh of the living.
For an extra dose of Nightmare Fuel, we have the Wracks, Dark Eldar who willingly let a Haemonculus turn them into lesser Grotesques. Some of them treat this as an apprenticeship, hoping to become Haemonculi themselves, but many are just bored with their decadent lives and want to spend decades as horrific abominations.
All of the Feldragons in Arc Rise Fantasia are heavily implied to be people of the Divine Race exposed to hozone without a dragon gem to protect them. Those that aren't Divine Race turned dragons are implied to be the results of Ignacy's experiments on Common Race people, including orphans.
Of course, since Hito-Shura (the "Demi Fiend") is you, and the transformation is mostly glowing lines and an aversion of shirts, the actual level of change may be as little as being purely superficial. Of course, now you're a Physical God, assuming that you aren't killed by the many, many powerful enemies running around. Or at least have a backup save. Which is good, because there are plenty of Gods trying to kill you.
Virtually all the monsters from the Resident Evil series (with the exception of a few based on animals) are humans infected with various viruses or parasites.
The ghouls in the Fallout series. Apart from looking like corpses, they're not really very different from humans (they're more resistant to some drugs and they may live much longer), but some go mad, becoming Fast Zombie-esque "Feral Ghouls". Super Mutants are an even straighter example, as they actively kidnap humans to infect with the FEV (Forced Evolutionary Virus).
A specific mutant in Fallout 3 actually says word-for-word "I was once a man" but he's referring less to his mutation and more to his being fused with a tree.
Dead Space. Every single enemy you fight was once a human; even the significantly larger bosses are made of dozens or even hundreds of human corpses joined together into one big mass. The Lurkers are encountered just one room after a datapad with a list of the month's births, confirming that yes, these were somebody's babies before they were murdered and transformed, just like the grown-ups.
The sequel introduces the Pack, a form of zombie made from prepubescent children who hunt in groups.
City of Heroes has several, notably the Devoured, the Hamidon, the Rikti, and as you find out in one story arc, Malta's Titan robots..
The Hamidon is definitely the worst case, being a giant Blob Monster.
Giant Blob Monster doesn't do it justice. It's an amoeba the size of a city block, created through some terrible fusion of genetic engineering and dark magic, with maybe some divine empowerment thrown in. Eldritch Abomination comes to mind.
The Broken, the Lost Ones, and the Eredar are all degenerated versions of the Draenei who were mutated by over-exposure to fel energy.
The Forsaken were the humans who were stricken with a plague of undeath.
World of Warcraft also inverts this when we discover that the Curse of Flesh caused mutation in many, if not all, of the Titan's creations. Earthen became dwarves, mechagnomes became regular gnomes, and Vrykul became Humans. These are all inversions since the transformations go from "monster" to man.
There is a theory that Night Elves (from which all other elves are descended) descended themselves from Trolls who were exposed to the arcane energies of the Well of Eternity, once more inverting the trope.
All but a small few of the demonic races were once regular beings that became so tainted by fel energy they eventually became full demons, usually caused by direct interference by the Burning Legion as an assimilation plot. Many night elves became full demons in the form of satyrs and many other races became part-way demonic such as chaos orcs and the felblood elves through similar corruption. The eredar are an odd case because the majority of the species became demons after siding with the Legion while a minority fled and became draenei, which is different because the previous cases had mainly the minority of their race become demonic.
Only a few races like the Nathrazim are confirmed to be originally demonic.
In Halo: Captain Keyes, when you eventually find him. He's been absorbed into a massive Flood organism, retaining enough sentience to contact you on your commlink through sheer force of will. Not to mention the fact that the Master Chief needs his neural thingywhatsit, requiring him to punch through what was left of his face into his brain. Hell, any human (or Covenant, if you consider alien races) that the Flood infects could be considered an example of this.
In Halo 4, we find out that most of the Promethean Knights used to be human, who were harvested by the Didact to be made into weapons.
Kokiri are said to turn into the race of skullkids when lost. They're usually shown as more tragic enemies, you being able to talk with them as a child in The Legend of Zelda: Ocarina of Time.
The same game mentions that Hylians would eventually be transformed into Stalfos when lost in the woods. Just as creepy, given that you actually meet a lost man in the woods who appears oddly gaunt and skeletal, and when you return from his fetch quest with what he wants, he's gone, and a Stalfos is waiting for you.
All the NPCs in the Dark World of The Legend of Zelda: A Link to the Past, as the Dark World has the effect of transforming anyone who enters it into a form that matches his personality. Link, who in this game is apparently a coward who must overcome his fear, is transformed into a pink bunny rabbit without any means of protecting himself.
Similarly, the Twilight in The Legend of Zelda: Twilight Princess turns normal humans into spirits, Link into a wolf, and does not seem to affect magic-users. Spirits (and Twili) will eventually transform into the inconsistently named "Twilit Messengers" (better known as "those screaming things that guard portals").
Though they aren't actually monsters, Ezlo and Midna probably count.
Done in at least two side quests in Final Fantasy Tactics A2. In one mission, a requester asks for help as he and his friends are trapped in a mine and is afraid he will become the ghosts that haunt the place. When you get there, you encounter a gang of ghosts and its leader weakly begs for help.
In another mission, a requester asks for a Potion and Hi-Potion to treat some wounds/fatigue. When you meet the person, it's actually a zombie, but has retained enough sense and control to talk to Luso normally. When the zombie uses the Potion on itself, it winds up hurting itself and Luso has to stop it from drinking the Hi-Potion. That's when the zombie realizes it is dead and was wondering why clans were attacking and people at inns throwing rocks. Later on, you discover that the zombie is actually Frimelda, a former Blademaster. She fought battles with a Paladin and over time, he grew jealous of her success as a fighter while he failed to follow in her footsteps, so he drugged her and she became a zombie. You can heal her eventually and she will join your clan.
One of the gifted somehow turned into a dragon, originally being a hume.
In Golden Sun, the people of Bilibin wanted to build a castle out of wood for their queen. The tree that they put their axes into turned out to be sentient, and cursed all of the villagers of Bilibin and Kolima to turn into trees. Fortunately, this gets undone after his Heel-Face Turn.
Hooray, we defeated the dragons! Wait... oh, crap.
Subverted in the next game, when Siegfried is human again, and his armor lives on as the new Nightmare.
And, of course, Lizardman.
Raphael becomes infected with Soul Calibur's evil and becomes some sort of vampire-like entity.
Cervantes as a one time wielder of Soul Edge has been twisted into a ghost/zombie pirate.
Charade and SCII Berserkers also count.
Algol is another example.
Not literally a 'man' but close enough, Scorpiton from Patapon. He actually is Makoton, who sold his soul to a devil to avenge Aiton, a Zigoton you killed during the first half of the game.
Dr. Nefarious from Ratchet & Clank, and despite the fact that he was once human, he hates humans with a passion, calling them "squishies" and other derogatory names.
Stalkers in Half-Life 2 are humans taken prisoner by Combine forces for acts of rebellion or "simply being in the wrong place at the wrong time", according to Alyx. Their organs and genitalia are removed and their blood replaced with a saline solution, a metal plate welded over their "face" and a strong implication that they are rewarded for good behaviour with artifical limbs so that they can walk. Brrr.
For that matter, the Combine soldiers themselves were once humans who have undergone memory replacement and had most of their organs replaced with cybernetic implants. The soldiers rise in rank by giving up more and more of their humanity. It's implied that all of the Combine's bio-weapons had similar origins.
Final Fantasy X does this a lot. All the Aeons were human, and an optional superboss is a monk named Omega whose hatred of Yevon turned him into a gigantic, four-legged monster with the power to create novae.
The most Disturbing / Saddest example however is Anima, who not only looked monstrous (a chained, gap-mouth giant corpse rising out a shell) but is Seymour's human mother, who basically had turned herself into one to give him the ability to return to his father's people. Now she rages at how evil he became.
The fiends you fight in random encounters were once human souls. In fact, in the cutscene where the party reunites with Yuna in Home, you can see fiends forming from the souls of recently killed Innocent Bystanders in the background. And, of course, to top it all off, there's Sin, aka Jecht.
Pretty much everybody in Nexus War: Demons, angels, undead, and several others. All were once human and have become utterly inhuman. Just about the only characters that don't fit this trope are Eternal Soldiers.
StarCraft's Infested Terrans, which are a lot more open about it (since they're named, well, "Infested Terrans"). They're still horrific abomination suicide bombers, though.
Also from Heart of the Swarm, the unique Zergform Izsha, who serves as Kerrigan's equivalent of an adjutant and who can be charitably described as a "chitin-plated naga with claw-tentacles for hair", is eventually revealed to have been a Terran woman, a medic named Amanda Haley, who was mutated into this new form by the Queen of Blades when she sensed the woman's psionic potential.
In Knights of the Old Republic, rakghouls all used to be human before being bitten and infected. The player gets to meet some Outcasts and a terrified Republic soldier before they transform. Other parts of the EU use the rakghouls again, only this time there's a deranged Dark Jedi with a talisman that instantly transforms non Force-Sensitives into them. An old clone trooper holds out longer than most, but doesn't quite manage to kill her.
Also invoked to describe the unusual nature of Darth Nihilus. Although he's been implied to be a Negative Space Wedgie, or even the hero's Enemy Without. All depends on who you listen to.
Although not originally human, Sonic's were-hog form in Sonic Unleashed can be an example of the original mind/new body version.
The Archie comics give us a much straighter example in Jules.
Many Neopets characters like Edna the witch (now a green Zafara) and the Island Mystic (now a yellow Kyrii).
Rapture's Big Daddies are revealed to be this in BioShock.
The trope's name was used word-for-word in Prince of Persia: The Two Thrones by the Prince to describe the giant, jawless boss he has to fight. And just like said boss, most of your enemies from the first and third game from the series are this.
The Many in System Shock 2. It's really unnerving to having your enemy viciously assault you while screaming for you to run away or put them out of their misery.
Also, if the Chantry's teachings are true, the first Darkspawn were this.
The Dragon Age II DLC Legacyconfirms them. The final boss of Legacy, the Awakened Darkspawn Emissary Corypheus, was one of the original Tevinter Magisters who tried to claim the power of the Golden City.
Dawn of War: Dark Crusade invokes this with Macabee, an archaeologist who awoke things best left undisturbed and was turned into a Necron Pariah. When another race assaults the Necron stronghold, Macabee notes "I was like you, once, clinging to life and blind to the truth... deep in these catacombs, I was remade..."
In the Diablo games, Humans possessed and altered to fit their shape by the Prime Evils, through Demonic Possession. Diablo's body turns back into that of the young prince in the first game when he's killed, and in the second, all the Three Evils are in the bodies of possessed humans, which turn more and more monstrous in irregular stages.
Parasite Eve 2 is a more nightmare fueled version than usual: every single one of the monsters in the game were people who worked on the "Second Neoteny" project, which was a project intended to transform large numbers of humans into monsters. Humans who VOLUNTEERED for the project. This makes the monsters even more horrifying, since once your attention has been called to it, you can easily see how they were altered from their original human form.
Made even worse when it is revealed they used the Heroine's DNA to make start the process to begin with. So even the monster-part came from a human as well.
In Wario Land 3, all the monsters in the game were humans transformed by the Hidden Figure.
Phantasmagoria: A Puzzle Of Flesh reveals that the Hecatomb was a human beingnote To be more specific, the real Curtis Craig that transformed into a monster - one that retains only vaguely human-like characteristics.
The Black Knights in 11eyes underwent an Emergency Transformation courtesy of Misao. As Misao is the only one of them technically alive, she's also the only one that can change back. It doesn't help that they're wearing the monsters of the Red Night as a sort of living armor.
In King's Quest VI the Lord of the Dead was a man long ago, chained to the throne of the Underworld and enslaved as its ruler. The throne and his witnessing of unending tragedies slowly drained away the man's humanity and transformed him into something beyond comprehension.
In Portal 2, it's revealed that GLaDOS is less of an artificial intelligence and more of a human one; having been the product of Brain Uploading the consciousness of Cave Johnson's secretary, Caroline, into a computer intended to house Cave's.
In inFamous 2, the swamp monsters were once humans, transformed by Bertrand. Bertrand himself can also change into a gigantic monstrous form.
In the Game Boy Advance and PSP versions of Final Fantasy IV, Cecil's trial in the Bonus Dungeon includes a potential encounter with a Goblin who insists that he used to be human, and that the curse on him will wear off shortly. Indeed, if you don't attack him, the battle automatically ends and a man is standing where the Goblin was (killing the Goblin causes Cecil to fail that part of the trial, of course.)
All of the main characters in Final Fantasy XIII are normal humans who were turned into l'Cie (servants of fal'Cie, the providers for Cocoon/Gran Pulse), which grants them strength and magical powers, as well as resulting in them being ostracised by the people of Cocoon. Also, it is revealed early on that any l'Cie who fail their task are turned into Cie'th, deformed crystal-covered monsters who wonder the world until they eventually lose their will and turn to stone. And even then they are still alive, and awake and suffering.
Version 6 of Ao Oni has all three of the four main characters converted into one of the monsters when they were killed.
The Elder Scrolls V: Skyrim has the Falmer, former Snow Elves who after losing a war against humans sought refuge with the Dwemer, a technologically-advanced race of underground elves. Despite their early hospitality, the Dwemer suddenly turned on and enslaved their kin, feeding them strange mushrooms and torturing them for their amusement. Not only did the Snow Elves degenerate into something like Morlocks, but what the Falmer went through fundamentally changed their very souls; theirs can be trapped in normal soul gems like any other beast, while sentient beings like humans or elves require black soul gems. The Dwemer literally devolved the Falmer into animals.
A major part of the Reapers' modus operandi in the Mass Effect series. It begins with their Husk grunt troopers in the first game, which are augmented by variations and husks made from alien species in the others. But the crowning achievement is the revelation in the climax of the Suicide Mission that Reapers are made by pasting up sapient species and implanting them in metallic superstructures — and their next target is humanity.
The Collectors in Mass Effect 2 were originally Protheans the Reapers transformed into slaves.
In Mass Effect 3 basically every major sentient race (save the quarians) gets their own monster version that they can be turned into. Add a couple of cross breeds for the charm.
In Little Samson, Kikira the Dragon Lord, Gamm the Rock Lord and K.O. the mouse were all human once, but were transformed for various reasons. This is All There in the Manual and is nowhere brought up in the game.
X-COM: Terror from the Deep has the Bio-Drones, whose UFOpedia entry reveals that they're actually made from living human brains that had been tortured to submission.
The ultimate villain of the Baten Kaitos games, Wiseman, is implied to have been a human sorcerer who desired power so much that he abandoned his own humanity to obtain more.
Demons in Dominic Deegan Were Once Men; we get to see some of the transitions. Most notable, of course, is Siegfried, whom we knew rather well before his death. ( In fact, he was the second recurring character to be introduced.) From this to this to this.
Then there's Karnak, who is revealed to have been human near the end of the Ecstasy & Evil storyline, very casually by a former friend who has apparently given up even being sad about losing him. His back story is fleshed out as the comic progressed, and so far the only really bad thing he ever did in his life he repented halfway through and ran away to sacrifice himself heroically. May be the only being in hell not rightfully damned there.
Demon!Karnak, however, is both evil and a total Jerk Ass, although he has been having some character development. Had a Rorschach Moment in December 2010 and appointed himself Warden of Hell and informed the damned that they were all trapped in there with him.
Karnak: [dramatic slaughter montage, final panel thought bubble] I hate this place.
A more typical progression of this is probably provided by Lady Loxo, who used to be soulbound to the Demon of Treachery back before Karnak exploded Hell into submission, and who becomes all serpentine and scaly after beginning to consume souls out of the 'feeding pits.' Bulgak Adrak is much more conflicted and his transformation doesn't progress nearly so smoothly, even once he gives in. Then he has an epiphany and his soul explodes.
Then there's TIM, the eyeless, nameless infernomancer who provided the first evil in the comic and kept coming back with new levels in Body Horror.
This trope is not applied to benevolent synthetic entities like Quilt and Acibek who were made out of unwilling human victims. Political Correctness Gone Mad, possibly, but while the Acibek thing was tragic and the Quilt thing creepy, the focus is intentionally on who they themselves are, not their antecedents.
Jacob Deegan, in his extremely long 'quest to become The Zombie Alive' phase, is explicitly trying for this effect as hard as he possibly can. The universal Squick is a perk.
Addressed in The Fancy Adventures of Jack Cannon; as a combined punishment-and-strategy, Frankie is turned into a monster, shaped roughly like his old human self, but bigger, stronger, and uglier, with glowing red eyes. Gavin remarks that the eyes were a mistake; they made him too monstrous, making him easier for Jack and his family to deal with. Gavin restores his original human eyes so that the next time they make him fight someone, his opponents will be thrown off balance.
Web Serial Worm features Mannequin, once a meek Gadgeteer Genius striving to avert the Reed Richards Is Useless trope, now a member of the villain group called the Slaughterhouse Nine, encased in a special protective suit of armor that features retracting blades, unnatural ranges of movement, and detached limbs that he can swing, throw or fire from his suit using chains. Topping it off, he may only be a brain and a few organs that fit in the armored torso. And he did it all to himself. The Endbringers might fit this trope as well.
Pretty much all the minions of supervillain Dr. Macabre in the Whateley Universe. He kidnaps teens, transforms them into monsters, and forces them to do his bidding. The one captured by the police is a girl who is now mostly ghoul: she has a mouth full of needle-sharp teeth and requires human flesh to survive, despite everything she has tried.
The creepiest part of Ben 10: Alien Force is the revelation that the DNAliens were all once humans who had brain slugs put on their heads. It's never really addressed after "Max Out", which was the most serious and darkest episode of the entire series, though.
Except for one where an amnesiac man who can only recall being taken by the aliens turns out to be a disguised DNAlien and is restored to being a human again at the end of the episode.
In one episode of Invader Zim ("Gameslave 2"), the rat people in the labyrinthine parking complex Dib gets lost in claim this happened to them, but Dib is unconvinced. A female one actually says "I was once a man," causing Dib to respond, bewildered, "But... you're a woman."
Also, from "The Sad, Sad, Tale of ChickenFoot", we have this:
ChickenFootI was once a man, like you! I once worked in a Chicky-Licky hut, like you! Dib: I don't work in a Chicky-Licky hut. ChickenFoot DON'T LOOK AT ME!
For those not in the know, ChickenFoot is actually just some guy in a chicken suit, having problems with the zipper.
Parodied in Sealab 2021, in which a talking tree cobra claims "I was once... a man!" before saying "Just kidding, I've always been a snake."
After Cobra Commander gets hit with altered fungus in G.I. Joe: The Movie and starts turning into a snake, all he can hiss is "I was once a man!" until the transformation is complete. Definitely the Trope Namer and what Sealab referenced in the above quote, despite not being "human" in the first place in this continuity.
Although he got somewhat better in the series immediately following the movie with some assist, which justified in story the Power Armor he wore.
Windfang, Wrath-Amon's literal Dragon from the animated series Conan the Adventurer. Once a famed general, he was taken prisoner during a battle in Stygia and transformed by Wrath-Amon. His wife-to-be ran from him in horror, leaving Windfang with nothing but 200+ years of servitude to the evil wizard. Wrath-Amon's second lieutenant Skulkar also was a human before a double whammy of sorcerous empowerment and necromantic curse turned him into and undead horror; unlike Windfang, though Skulkar definitely prefers being a monster (the episode with his backstory is also the one where he gets temporarily remade human, which horrifies him).
On the heroic side, Greywolf's brother and sister also were cursed into wolflike monsters under a villain's control (although fortunately by the end of the episode their curse was mitigated into them being intelligent free-willed wolves for most of the rest of the series). Also, in one episode where Conan gets sent to a Bad Future, we see what happens when Jezmine's serpentman heritage is brought to the fore; she later turns into a giant snake in a Heroic Sacrifice to hold back Set so Conan can escape him - and then Fridge Horror hits you as to exactly why she would have that ability when she screams at Set as she "scales up" that she'd never let him touch her again...
Generator Rex. After the Nanite event, every living thing was infected by nanites. Many EVOs were human at one point. And any one can go EVO at any time. And the only one who can cure them is a teenaged boy, and sometimes not even then.
About half of the mutants in Street Sharks, with the other half being sea creatures-turned-mutants. The good news is that the mutations pretty much can't change one's free will, so fewer humans are gradually transformed by the Mad Scientist.
Not true...the bad guy's working on a formula to make them under his control. He just keeps having the test subjects escape...
In an episode of Futurama, Leela encounters an octopus-like creature in the sewers that claims (in a deep, growly voice) that it "used to be a little blonde girl named Virginia".
In Teenage Mutant Ninja Turtles (2003), the Turtles find an underground community of monsters who were once human, victims of the Foot Clan who were transformed into these creatures to mine valuable minerals. They get lucky, however; it takes many episodes, but Donatello is eventually able to find a permanent cure.
Of course, since mutants are one of its main concepts, the trope appears in some way, shape or form in pretty much every iteration of the franchise.
In "Finn the Human", it's revealed that the Lich was apparently a normal human who was mutated by the Mushroom Bomb's fallout into the undead Omnicidal Maniac we all know and fear. In "Jake the Dog", the alternate universe Jake is transformed into the Lich instead.
In an episode of the 1990s Silver Surfer animated series, the Surfer and a group of researchers come across an enormous green blob monster on a universal library planet built by Precursors. It's the precursors (and the crew of a pirate ship) themselves after they devolved into this form and linked up with each other in a hive mind of knowledge.
Played for Laughs in the Looney Tunes short "Mutiny on the Bunny". A horrid-looking and clearly insane man runs down the gangplank from Yosemite "Shanghai" Sam's ship and has just enough time to turn to the audience and declare "I was a human being once!" before running off screaming into the night.