Buffy the Vampire Slayer: Vampires are humans who have been turned into soul-less demons. Some of them, like Angel and later Spike, are able to regain their soul, but others are quite better off dusted. Also, restoring the soul doesn't reset a vampire to the person they were as a human. It just makes them capable of feeling empathy and guilt for what they'd done in the meantime.
In Chrysalis, the Terran is aware from their "birth" that they were once human, complete with memories as a human, and they seek to preserve their humanity through many methods even as they build themself a spaceship body in their quest for revenge, chief of which being their refusal to make restorable backups of their consciousness.
The original Daleks started off as the Kaleds of Skaro, who looked like humans (on the outside, at least), but started mutating into tentacle blob things due to a generations-long nuclear war. Then the chief Kaled scientist Davros realized this, and decided to speed up the process, removing a few little details like "empathy" or "pity" while he was at it.
Another Dalek faction, the Imperials, were made from Human Popsicles. "Not pure enough in their blobbiness" indeed.
One iteration of the Daleks 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.
"Asylum of the Daleks": Oswin Oswald. The Daleks turned her into one of them, and she was unable to deal with it, instead dreaming that she was still human.
The Cybermen, more than any of the others. It's arguably the whole conception behind the way they were originally written in the 1960s.
"The Age of Steel": In the new series, 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.
"Utopia"/"The Sound of Drums"/"Last of the Time Lords": 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.
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.
Pretty much the whole cast of Doom Patrol (2019) but particularly former race car driver Cliff Steele who, after an accident, the only thing they were able to save was his brain and lives out his days as a robot.
The "Oathkeeper" episode reveals that a good number of the White Walkers were originally Craster's sacrificed sons whom the Night's King transformed with a touch.
The White Walkers attempted to turn Benjen Stark into a wight, but he was saved by the Children, who restored his mind. However, his appearance and the way their magic works makes it clear he isn't strictly human anymore either way.
The Night's King was a man before the Children of the Forest turned him into the first White Walker.
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.
Lost: The Man in Black 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. A flashback episode shows that this is true.
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...
Star Trek: All Borg started as other species, usually humanoid. Seven of Nine, in particular, is an actual human from the Federation.
Supernatural: Several of the monsters are transformed humans, the most important being the demons, which are made by torturing souls in the bowels of Hell.
That Mitchell and Webb Look: The Quiz Broadcast sketches have Them, deep-voiced, red-eyed zombie-like things who want to get inside (they really want to get inside), and are implied to know more about what caused the mysterious "Event" than anyone else. But they were once human, something that breaks the game-show host's Stepford Smiler routine briefly.
Host: Why do They look like us? Peter: Because they used to be us, didn't they? Host: Yes, that's right. They used to be... us.
In Episode 23 of the original series, the Monster of the Week is Jamila — an astronaut who was stranded on an alien planet without any water, only to be abandoned by his people back home. Mutated by the environment, the astronaut evolved into a monster and eventually rebuilt his rocket to travel back to Earth, which he sought to attack in revenge for its people leaving him out in the cosmos to die. Serious Tear Jerker ensues as our heroes are extremely reluctant to kill the mutant human, but are forced by their superiors to treat him as just another mindless kaiju.
Jamila is later homaged by Ultraman Gaia with a monster named Tsuchikera. He was originally Kondo, a biological warfare researcher during World War II whom the Imperial Army mutated with a serum he developed to create super-soldiers when he refused to use it for the war. He fled to the swamps driven half-mad by his transformation but his friend Hirano remained by his side to help keep him sane. Unfortunately, radioactive leakage from underground nuclear testing begins to scramble Tsuchikera's brain and turns him into a giant monster. The results are an even bigger Tear Jerker than Jamila's episode.