You see, in the mind of the audience (or at least in the mind of some characters) monsters are often initially assumed to be male. This trope covers situations when these characters are surprised to find out that the monster is, in fact, a female.
This takes the form of a monster in some way having her gender revealed, and someone acting surprised by this.
- Inverted in Slayers: a female-looking firebreather is revealed to be a "drag"-on despite Tertiary Sexual Characteristics, when Gourry gets a good glimpse of the underside.
- A chapter of Naruto did pretty much the same thing with a giant armadillo whose gender Naruto and Killer Bee argued about trying to discover, except that we ALL got a good look at its underside. TWICE.
- Combined with Samus Is a Girl in Negima! Magister Negi Magi: a Bounty Hunter with lecherous intent to several young girl turns out to herself be a Little Bit Beastly girl whose tentacles and stocky build come from a set of Powered Armor. She's still a huge pervert nonetheless.
- While it does not happen in Yu-Gi-Oh! ARC-V, White Wing Magician, the human version of one of the Four Heavenly Dragons, reveals that Clear Wing Synchro Dragon is actually female.
- In Pokémon the Series: Diamond and Pearl, Jessie manages to capture a Yanma, which then evolves into a Yanmega, a huge vicious-looking dragonfly with an extremely deep reverberating voice. During a later episode, a female Togepi uses the move Attract (which only works on targets that are the opposite gender of the user) to immobilize the main cast's Pokemon, including Pikachu, Piplup, Croagunk and Meowth (all four of whom are male). Jessie's Yanmega remains unaffected, and after putting two and two together, Jessie's shocked to find out that her giant bug Pokemon is, in fact, a female.
- In Marvel Comics, Piranha (a normal piranha fish mutated by radiation into a semi-humanoid being) was always assumed to be male since the character's introduction in 1974. Even A.R.M.O.R.'s file on Piranha lists "male" under sex. In a 2009 comic it's discovered it's actually a female when the monster has incubated humans with its offspring.
- In Popeye's "Plunder Island" Arc, Wimpy is at one point dragged off of Popeye's ship and onto the Sea Hag by the monstrous Goon. Soaked from being in the ocean, Wimpy decides to take off his clothes while being held captive with the Goon watching him. Just as he's about to strip naked, a voice comes through the door, saying, "Keep a close eye on him, Alice." Wimpy quickly realizes who 'Alice' is, and quickly re-clothes himself, saying, "I didn't know there was a female present."
- In Shrek, Donkey is confronted by a dragon on a collapsing walkway and attempts to smooth-talk his way out, before realizing that the dragon is female, and she thinks that he's hitting on her. He goes with it.
- Everyone assumes that Kevin is a boy bird in Up. Not unjustifiably — with many bird species the colorfully-feathered one is the male. (Peacocks are the classic example.) Kevin's plumage is modeled on a male Himalayan monal from the Sacramento Zoo. Kevin could still be male, even if he's raising babies, as there are several species of bird where the male does the childrearing.
- Strange Magic: Thang and the audience are both surprised to learn that his fellow goblin Stuff is female. She thought it was obvious.
- In Godzilla (1998), Nick is shocked when a pregnancy test for humans reveals that the giant lizard destroying New York is pregnant. It's subverted in that everyone still refers to Zilla as a "he" due to it being capable of asexual reproduction.
- In Dragonheart: Vengeance, Lukas is quite surprised to find out that Siveth, the Dragon he's been searching for, is female.
- Inheritance Cycle:
- In Eragon, Eragon comes with a long list of names to suggest for his newfound dragon to call her. He lists quite a few, before figuring out that the dragon is female, and he'd only suggested male names thus far.
- Another instance is seen in Brisingr with the Ra'zac. One is revealed to be female, while the gender of the other is left ambiguous.
- In Guards! Guards!, the dragon tormenting the city is revealed at the last minute to be female, to the surprise of the city watch.
- "Chelonauts" in The Light Fantastic are trying to go over the edge to determine the sex of Great A'Tuin, the turtle the Discworld travels on. Their mission is largely unsuccessful, but at the end of the book, the strange lights wizards have been seeing in the sky turn out to be Great A'Tuin's eggs, and hatch.
- In Harry Potter, Norbert the Norwegian Ridgeback is in the last novel revealed to be a Norberta, to Hagrid's bafflement. According to Charlie, the easiest way to tell this is that female dragons are even more vicious. Slytherin's Basilisk is also female, since it doesn't have a scarlet plume in its head like male basilisks do.
- The title character in IT is protecting Its eggs, which will presumably hatch into Mind Screw monsters just like her.
- In I've Been Killing Slimes for 300 Years and Maxed Out My Level, after Azusa defeats the dragon Laika, she demands that Laika make up destroying her house (on accident) during the battle. The thoroughly cowed Laika asks to return home first to retrieve reparation money from their Dragon Hoard. The next day, Azusa is shocked when a cute girl in Elegant Gothic Lolita clothing introduces herself as Laika in a humanoid form. The anime adaptation supports the deception by applying a distortion to her voice which makes her sound male, which disappears after the reveal.
- Animorphs: In The Change, Tobias and Rachel are surprised to hear one of the escaped Hork-Bajir (an alien race of 7-foot-tall, bipedal lizards covered in blades) call the other his wife.
- In the short story 'Labyrinth' Admiral Miles Naismith accepts the job of putting down a defector's Super-Soldier experiment, bungles the infiltration of his old lab badly enough for the guards to cast him into a basement to be devoured by said experiment, and... discovers a lonely teenaged girl.
- The Way of Kings (2010): Dalinar Kholin realizes about halfway though the book that the Parshendi send their "femalen" into battle alongside the males. Note that for Alethi like Dalinar, a woman fighting is about as scandalous as that same woman having sex in public. With someone she's not married to.
- Inverted in Jurassic Park, in that all the dinosaurs in the park have been engineered to be female, to prevent uncontrolled breeding and overpopulation. The "surprise" comes when Dr. Grant finds a nest of eggs with baby dino prints leading away from it, meaning that some of the dinosaurs must be in fact male. Justified by the use of frog DNA, as some species of frogs can change sex in a single-sex environment.
- The trope proper is referenced earlier by the park's chief geneticist, Henry Wu, who says everyone always refers to the adult Tyrannosaurus rex as a "he" despite being aware that it is female, like all dinosaurs in the park are supposed to be.
- In Reaper Nina starts giving Ben some compliments that perturb him coming from her rather masculine seeming demonic form. They later start dating, though mostly in her human form.
- Lorne's mother in Angel was played by an extremely muscular, bearded man. Angel is comically baffled when Lorne goes, "Hi, Mom".
- Star Trek: The Original Series: The Horta in "The Devil in the Dark" turns out to be female...and the "silicon nodules" that miners on its planet are destroying turn out to be her eggs.
- Night Gallery: In "The Caterpillar", a man pays to have a romantic rival murdered by having a carnivorous earwig placed in the man's ear. The plan goes wrong, however, and the next morning he awakens to find that the earwig has been placed in his ear instead, and has crawled inside his head. He endures weeks of agony as the earwig eats its way through his brain, but he remarkably survives the ordeal. He is then told by a doctor that the earwig was female... and it laid eggs.
- One 2E supplement about the Dragonlance D&D setting has a section on minotaur society, complete with an in-character narrator who'd been captured by sea-going minotaurs. When this character asks the ship's captain about minotaur family life, the captain gruffly points out that the husky, wide-horned first mate who's been holding the narrator's hands behind his back throughout their conversation is, in fact, his wife. This was in sharp contrast to the few pieces of artwork depicting minotaur females, which tended to be unsubtle about things.
- The gender of the giant featured in the second act of Into the Woods isn't specified until the characters actually see her, whereupon the Witch yells "THE GIANT'S A WOMAAAAN!"
- The dragon Hooktail in the English version of Paper Mario: The Thousand-Year Door.note Despite the character being the first major boss of the story, you don't learn this until the final chapter—where her older brother is the first of many bosses, and is predictably pissed off at you for what you did to his sister. Even the characters assume Hooktail to be male, and refer to her using male pronouns. This is averted in the Spanish localization, in which Hooktail is actually male.
- Psychonauts: Raz discovers that the lungfish residing at the bottom of the lake is actually "Linda". Although with the deep, booming voice, the audience can be forgiven for its assumption.
- Shale the golem in Dragon Age: Origins is revealed to have once been a female dwarf. Notably, even Shale is surprised by the revelation.
- You know the Level 300-ish dragon in Disgaea: Hour of Darkness that hangs around Laharl's throne room and helps save your ass once? She'll have you know that she's one of the more attractive of her lot. Dark Hero Days features a lesser surprise in the form of Actress, a female Entei. It would've been more surprising if not for the fact that you can hear her obviously female voice in the battle that occurs prior to someone bringing up her gender.
- In DuckTales Remastered, the Yeti boss of the Himalayas is shown to be a female. It's a reference to an episode of the original TV show, "The Lost Crown of Genghis Khan", which also featured an abominable snow-woman.
- The "surprise" was enhanced due to it being a blatant retcon (since she was originally referred to as "he" before the text on her card was changed later) but Shikana, Who Demands Tribute from Infinity Wars is actually female. She looks like this.note
- Hanna Is Not a Boy's Name has a vampire bat who Hanna wrongly assumes is male, and he seems to get more apologetic than she got offended when she corrects him. She has Tertiary Sexual Characteristics upon closer inspection.
- The Inexplicable Adventures of Bob!: Upon encountering Nessie the Loch Ness Monster, Bob says he thinks she's a girl monster, because she has eyelashes. Molly, being a monster herself, confirms that only girl monsters have eyelashes.
- In Rhapsodies Hilda and the rest of the Krampusi who show up in the Christmas episodes.
- Monster may be stretching the term slightly, but in the SCP Foundation, there is Dr. Wondertainment
- Dodoria in Dragon Ball Z Abridged, she was a male in the source material but to Vegeta's shock and disgust, she's revealed to a woman here, even regarding herself as the most beautiful (and fertile) on her home planet (until Frieza blew it up). Vegeta then proceeds to kill her and "repress the truth" of what he had just learned.
Vegeta: "Any last words before I kill you?"
- Short and [far from] sweet in Kai Abridged, a compressed summary of a season of DBZ Abridged:
Dodoria: "I have a Thick. Meaty. Vagina!"
Vegata: (blows Dodoria to shreds; then starts vomiting)
- This, in of itself, is a reference to the Hungarian Dub of Z, which made Dodoria a woman for some odd reason.
- The episode "Cammy Island" from The Marvelous Misadventures of Flapjack has the titular hideous ogre turning out to be a girl, who is implied to, at least mentally, not be that much older than Flapjack himself.
- Subverted/Inverted with Bigfoot in The Venture Bros. who Steve Summers was in love with. After Brock shaves Bigfoot for the sake of a disguise, he reveals to the audience that he's actually male.
Steve: What are you, shy? Sasquatch doesn't have anything you haven't seen before.
Brock: Sasquatch is something I haven't seen before!
- The Grim Adventures of Billy & Mandy: The show itself seems to have a hard time concluding whether Jeff is a male giant spider or a female giant spider as he has been shown to lay thousands of eggs in two episodes but has a masculine voice and married the Spider Queen Velma Green. Lampshaded by Billy.
- Green Lantern: The Animated Series has the main characters defeating a giant monster in what seems to be a gladiator-style fight (their translators are offline). However, then the spectators rush out to tend to the beast, and we hear them speak legibly for the first time.
Alien: What have you done to Mother?