Human All Along

"Dr. Joffries had the equipment at CHESAPEAKE to do something that would have come very naturally to any researcher: he could sequence the worm's DNA. Shortly after receiving the results from the DNA sequencing he sent a very short email to his colleague and EFRE head Dr. Paul Two-Horses. "Paul, it's us.""

- The View From Below.

These are cases where a strange or fantastic creature eventually turns out to actually be a member of homo sapiens - a human being - or a Human Subspecies.

Since humans make up the majority of fictional characters, and being a human isn't (normally) regarded as a strange character attribute, this is often used as an inversion of traditional reveal tropes (such as Not Even Human).

Often, the humans initially appear inhuman because of a Baleful Polymorph, or because of being some form of transhuman.

Anime and Manga

 * The Jovian lizards in Nadesico turned out to be humans.
 * Much like the above example, from Gundam AGE are actually members of Vagan, a human nation formed by colonists who were abandoned by the Earth Federal Forces 150 years before the start of the series.
 * In Eureka Seven, Anemone's appearance and role (and possibly her name) seem to imply that she is a Coralian. It turns out she's just a human whose appearance and abilities have been modified by horrible military experiments.
 * In Claymore, the youma turn out to be humans infested with parasites born from the flesh of the Descendants of Dragons.

Film

 * This is one of the twists in Pandorum, the creepy alien mutant cannibals are descended from humans who've evolved to survive in a foodless spaceship over thousands of years. It helped that their ancestors were dosed with a treatment that caused accelerated evolution.

Literature

 * In Dragonback, the long-lost original hosts of the K'da were called the Dhghem. Turns out that's actually "Human" in a really old language.
 * The koloss and kandra from Mistborn both turned out to be transformed humans.
 * In The Hobbit, Gollum is described as a strange and nasty creature of undefined origins. In Lord of the Rings, we get to know he's in fact a hobbit whose appearance was twisted by centuries of exposure to the One Ring.
 * Alastair Reynolds' short story Merlin's Gun, the second with the title character, expands on the nature of the shadowy civilisation that was only said in the first story to have 'come from near the galaxy's core' -- they are cybernetic (and seemingly nuclear-powered and space-worthy) humans. The misinformation about their alien origins (although they could have more latterly come from the core, much time had elapsed) underscores the point of the story, which is about humans being shortsighted.

Film

 * In Rock-a-Doodle, Edmond was a Baleful Polymorph who had been turned into a kitten, but none of the animals really believed him until he turned back into a human at the end of the movie.

Live-Action TV

 * There's one episode of the original Twilight Zone, titled "The Invaders", where a woman is getting unpleasant visits from tiny alien invaders. But those people turn out to actually be human beings from Earth, and the woman's world is the alien planet, and the alien we've encountered is a humanoid giant.
 * An episode of the 1980s Twilight Zone had this as well: We also initially assume that "Kreetor" is a linguistic devolution of "Creature".
 * In an episode of Star Trek: The Next Generation, transporter-phobic character Barclay, when forced to travel via the transporter, sees some strange virus-like creatures in the matter stream. After suffering greatly due to his fears, he eventually figures out that those strange beings are actually humans, stuck in the pattern buffer.

Western Animation

 * Futurama: Leela claims to be an alien cyclops woman who has been trying for years to find her home planet. But she's wrong. She is actually
 * (Almost) every last goddamn Scooby Doo villain.