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.
Every demon in the Berserk universe was once a human. Humans become demons when a creepy little egg-like item called a Behelit comes into their possession and they hit an emotional nadir where they will do anything to get out of their current situation. At this point, the Behelit rearranges the features scattered upon its surface into a human face and screams, which summons the demonic gods of the Godhand, who proceed to offer him the chance to become a demon in exchange for the sacrifice of those closest to them. It also turns out that every member of the Godhand was also human, and they are created with the use of Behelits as well. While demons do sometimes retain the personalities they had when they were human, all too often they commit Transhuman Treachery, becoming Complete Monsters of the worst order, with many of them preferring to dine on their former species.
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.
She gets better, and in perhaps the Crowning Moment of Heartwarming of a very dark story, she is overjoyed when she realizes she can't do simple math.
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.
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 Alan 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 Alan 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 ablity 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.
Though he's not terribly monstrous or inhuman now, even in his demon form, and only really has to avoid it because the minus wave currently makes demons Ax Crazy.
In both the animes of Full Metal 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.
In Neon Genesis Evangelion, the EV As, which at first glance are giant robots, but are in fact giant, sentient, human-like beings, were all once human females, who gave their human bodies to become gods.
Comicbooks
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.
In Camelot 3000, Morgan la Fay keeps an ape-like animal on a leash as a Right-Hand Attack Dog. At one point, she informs an underling that it was once a man, until he got on her nerves.
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.
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).
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).
The mutants in the Doom movie. Condemned criminals who were injected with an extra pair of chromasomes, 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.
Freddy Krueger from the A Nightmare on Elm Street movies. Once a human serial killer, he turned into something resembling a nightmare god after his death.
Literature
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.
J. R. R. Tolkien's Middle-earth (The Lord of the Rings, etc): As individuals, there are the Nazgul (once human leaders) 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 corrution 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.
In The Relic, the museum beast is revealed to be a scientist mutated by an ancient retrovirus.
In William King's Warhammer 40,000 novel Space Wolf, Ragnor is most horrified about the nightgangers that they find in a Chaos-tainted cave because they, or their ancestors, had been human once.
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.
In Graham McNeill's Warhammer 40,000Ultramarines novel Dead Sky Black Sun, Uriel realizes the Unfleshed — monstrous, gigantic (next to him, a Space Marine), and flesh-eating — were once not only human but children. When they are willing to speak with him, having smells that he came from the same place they were made, he finds that they are still good and will help him in his his quest. One sadly confesses to him that they loathe themselves because of their forms. In The Killing Ground, Uriel must Mercy Kill the last survivor and is left deeply melancholy thereafter.
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. Then the planet fell to the Yuuzhan Vong…
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 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.
In Book 5 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.
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.
He was not mad, as a man is mad. He had dwelt apart from humanity so long that he was no longer human.
Also, Thak from "Rogues in the House." It's unclear whether he degenerated from humanity into his apelike form, or whether he ascended from apedom into something resembling humanity.
In the Gaunt's Ghosts novel His Last Command, the stalkers are Guardsmen and ogryns captured by Chaos and reshaped into beasts.
In Pat Murphy's There And Back Again, Rattler, the cyborg Gollum-equivalent, was human until the Resurrectionists started experimenting on her.
All the monsters having once been human is a major theme and source of conflict in the Felix Castor novels.
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.
Live-Action TV
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.
Let's not forget the Toclafane.
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, The Seeds of Doom.
Poor little Jaimie from "The Empty Child" / "The Doctor Dances". Thank Godhe got better.
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.
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.
In the cult One Episode Wonder series Heat Vision And Jack, the Cool Bike Heat Vision used to be Jack's friend Owen, before he got hit with an experimental ray gun that caused him to merge with his motorcycle.
Music
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.
In some Greek myths, Medusa was a priestess who was transformed as punishment for being raped by Poseidon in Athena's Temple (Athena, pissed that sex was going on in her temple, but unable to actually do anything to Poseidon, opted to punish Medusa instead).
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.
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.
Toys
The Bohrok in BIONICLEwere Once Av matoran. goes into Accidental Nightmare Fuel territory when you reailze that nuparu made the boxors out of Bohrok, so the boxors are made out of av matoran
They're people. Boxors are people! Sorry, it had to be done.
Tabletop Games
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. 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.
Videogames
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.
Virtually all the monsters from the Resident Evil series (with the exception of a few based on animals) are humans infected with various viruses.
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 zombie enemy you fight was once a human. The tentacle-baby monsters are implied to be embryos that were being vat-grown — possibly simply as an alternative to pregnancy, as it's demonstrated that the technology to grow limbs and organs independently exists in this universe
City of Heroes has several, notably the Devoured, the Hamidon, the Rikti, and as you find out in one stoyr 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.
This is fairly common in the Warcraft universe. Notable is the Demon Hunter Illidan Stormrage, who, after using a demonic artifact to gain power, was transformed into a demon.
Illidan's got nothing on, well, every race on Azeroth AND beyond. Firstly, those that play it strait:
The Broken, the retconned 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 War Craft 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 descended 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 interferrence 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 them 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 become absorbed into a massive Flood organism. And yet, retained enough sentience to contact you on your commlink only a few minutes earlier. 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 of the marines that flood infect could be an example of this.
Most, if not all, of the monsters from Clive Barker's Jericho.
The Legend of Zelda's 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 Ocarina of Time.
The same game mentions that Hylians would eventually be transformed into Stalfos when lost in the woods. Not as creepy though.
All the NPCs in the Dark World of 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 is transformed into a rabbit without magical protection.
Similarly, the Twilight in Twilight Princess turns normal humans into spirits, Link into a wolf (much cooler than a rabbit), 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.
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.
Stalkers in the Half-Life universe 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.
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.
Silent Hill 3, allegedly. "They look like monsters to you?"
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.
Not truly a monster, but in Chrono Trigger Frog (formerly Glenn) fits this trope. He doesn't let it get to him anymore.
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..."
Humans possessed and altered to fit their shape by the Prime Evils in the Diablo games, 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 being 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 6 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' AI was based upon that of Aperture Science's founder's secretary Caroline, and being disconnected from her mainframe causes a flood of memories to surge back into her about those days.
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.)
In Super Robot Wars Z 2: Hakai-hen, The Dimesional Beasts are people that were fused with their mecha by Gaiou. The Reveal comes in 49.
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 flesh-monsters who wonder the world until they eventually loose their will and turn to stone.
Version 6 of Ao Oni has all three of the four main characters converted into one of the monsters when they were killed.
At the end of Magical Girl Lyrical Nanoha As Portable: The Gears of Destiny, the Unbreakable Darkness, the Eldritch Abomination that the cast had been been fighting against for the entire game, is revealed to have once been a human named Yuri Evelvine.
Demons in Dominic DeeganWere 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.
According to his song, Trogdor was originally a man, but then turned into a dragon-man before finally becoming a dragon completely.
Western Animation
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:
ChickenFoot" "I 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.
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 guys' 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 an episode of the 1990s Silver Surfer animated series, the Surfer and a group of researches 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.
