Padre Bateleur: How do you sexnote such a dangerous animal?
Flinx: In my case, by dumb luck. She had babies.So you have a pet and you love him dearly. Let's call him George. One day, you notice that George has put on weight and isn't moving as easily as he used to. Worried, you rush George to the veterinarian, expecting to hear the worst. You're ready for just about everything except what you hear — your "George" is actually a Georgia, and she's pregnant. Oops. Of course you still love "him", but now what do you do? May go the other way: a pet thought to be female is revealed to be male. Obviously, if that happens, it isn't likely to involve a pregnancy, but you may begin to suspect things are off if she goes for a very long stretch without having babies. (And 'she' could of course be found out by getting another animal pregnant). Can easily be Truth in Television for rabbits, guinea pigs, reptiles, as well as cats. There are very few coat patterns exclusive to male cats * , and it takes some experience to be able to identify a cat's genitalia except in particularly blatant cases. The reverse situation is equally common - veterinarians find it just as frequent to let cat owners know that their cat is male when they thought it was female. Both situations commonly occur with rabbits and guinea pigs as it's hard to identify their genitalia as well. It's also difficult to sex a snake without sticking a metal prod up their cloaca (which should only be done by vets or herpetologists, as laymen risk hurting the poor snake). If it goes all the way in, it's a boy. If it gets about 1cm in before you hit something, it's a girl. If an actual tomcat is pregnant, that's Mr. Seahorse. See also Gender-Blender Name.
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- In ARIA (the manga, at least), President Maa is normally referred to as Maa-kun and thought to be a male cat, until the trio went to the vet for annual cat vaccine injection, which then reveals that Maa is female. They threw around the idea of calling her "Maa-chan", but by then they'd called her "Maa-kun" for so long that calling her -chan wouldn't have felt right.
- In a scene in the first episode of Shinkon Gattai Godannar!!, Anna is stroking her pregnant cat, and she states that she's surprised the cat turned out to be female. Her mother responds by saying she's surprised in all this time (five years) Anna hadn't noticed.
- In the long running Giles comic, the children get Grandma a parrot for Christmas. They name it 'Attila the Hun'. Attila later lays eggs.
- In The Life and Times of Scrooge McDuck, one of the stories involves Scrooge bringing two square chickens home to breed. They realized their error when the "female" chicken began to crow.
- Although not revealed in-story, this is presumably why May Parker owns a male dog called Ms. Lion in Lockjaw and the Pet Avengers. Apparently, she hasn't found out yet.
- Inverted in a Kellog's Raisin Bran Commercial. A man needs milk for his cereal and sees a cow in a nearby field. As he's walking out his housemates remark that it's not a cow it's a bull.
Films — Animated
- She didn't get pregnant, but in Rise of the Guardians, Bunny calls Abby the dog "he", despite being called "she" by her owners, and the fact that Abby is a girls' name.
- Kevin the giant bird from Pixar's Up, although the actual pregnant part is skipped over and we just see the chicks. The main characters keep calling her "Kevin" even after discovering that she's female, though they do at least switch pronouns. In fact, there was a subplot where Carl had to protect Kevin's egg from Muntz because it could make you grow young. Despite the interest the plot thread generated, it was dropped from the film for being "too bizarre".
- Kiara at the very beginning of The Lion King II is presumed to be male by Timon and Pumbaa. Also a sort of meta example: there was at least one tie-in book that talked about Simba and Nala's son, but the sequel only gives them one female cub. It's particularly confusing for fans, as Disney gave the lion cubs distinct sexual dimorphism in the form of a break in the light belly fur at the neck for the males. At the end of the original film, Simba and Nala's cub shows this break in the belly fur, indicating a male. By the next film, all indication or mention of the male has disappeared, and Kiara is shown with unbroken light belly fur.
- In the How to Train Your Dragon special "Gift of the Night Fury", after discovering that Fishlegs' dragon Meatlug has laid eggs, Ruffnut tells him "Your boy dragon is a girl dragon!"
Films — Live-Action
- The Disney live-action film The Three Lives of Thomasina has this as part of the Back Story, explaining the titular cat's name. Seems a little odd that this mixup occurred with a cat who lived in a veterinarian's home.
Thomasina: They started out by calling me Thomas, but when they, um, got to know me better, they changed it to Thomasina.
- In the Jerry Lewis comedy The Geisha Boy, the main character, a magician, has a rabbit named Harry. At the end of the movie it is revealed that Harry, is actually a Harriet.
"And I've been undressing in front of you for all these years??!"
- In Stage Door, Eve finds out at the end of the movie her cat Henry has just given birth to a litter of kittens. Terry suggests changing the cat's name to Henrietta.
Eve: I'll never put my trust in males again!
- In I Remember Mama one of the girls has a cat named "Elizabeth". Even when told that he is a tom, she refuses to change it.
- The book Thomasina (which inspired the above film) involves a female cat mistaken for a male. Her name was originally Tom. The cat's sex was revealed with age. Ironically, Thomasina's owner is the daughter of a vet, but he disliked the cat and didn't bother to check the sex for his daughter.
- The Baby-Sitters Club:
- There is a book where Jessi is petsitting and thinks that a fat old male hamster is sick, so she takes "him" to the vet...
- In a later installment of the Little Sister spinoff, Karen's class gets a second guinea pig they name Everett. A few people point out how fat Everett is until they discover "he" is going to have babies and change the name to Evelyn. The rest of the subplot is about raffling off the baby guinea pigs to kids in both Ms. Colman and Mr. Berger's classes.
- In the Harry Potter series, Hagrid's temporary "pet" dragon Norbert is mentioned in passing in a later book to have turned out to be female and redubbed "Norberta." Female dragons are More Deadly Than the Male. This explains a lot.
- In Sophie's Adventures Sophie adopts a stray cat, which she names 'Tom' and which promptly gives birth to four kittens. On ringing up her Great Aunt for new name suggestions, she's horrified to hear the suggestion of Tomboy ("That's worse than plain old Tom!") until she finds out what it really means.
- Later, she names three of the kittens Molly, Holly and Polly, but it turns out they are toms, and "Polly" gets renamed Ollie.
- In Diary of a Wimpy Kid Hard Luck, Greg's aunt Gretchen's rabbit is thought to be a buck, but she has babies.
- In Delilah Darling Is In T He Classroom, Delilah babysits the hamsters Polly and Dolly, but they have babies as it turns out Polly was actually a Wally.
- In Thursday Next, everyone is quite surprised when the title character's pet dodo, Pickwick, lays an egg. Fortunately, the name is already gender-neutral.
- Happened to the main character in the Papelucho book series and his sister. They adopted a cat whom they named "Teodoro", and later it turned out Teodoro had to be renamed as "Teodora" (female version of the same name) by the children since she just had a litter.
- In Robert A. Heinlein's Farnham's Freehold, Barbara is quite surprised to find that the cat, named Dr. Livingston, is pregnant, since she'd believed the cat to be male. The five other human members of the group have known the cat a lot longer than Barbara has, and consistently use male pronouns, which probably adds to the confusion.
- Bill (later Billina) the chicken from Ozma of Oz and later Oz stories.
- In Alan Dean Foster's Pip and Flinx novel The End of the Matter, this trope happens with Flinx's Alaspinian minidrag, Pip, after a "homecoming" trip during which "he" socializes with a definitively male native minidrag. It also introduced a subtle continuity error with the earlier novel Bloodhype, which was set several years later in the series's timeline but referred to Pip as male.
- L.M. Montgomery's Rilla of Ingleside contains a cat the Blythes call "Jack Frost" and think is male until it has kittens. (They still refer to "him" with the masculine pronoun, though, since it's become habit by this point.)
- In Debi Gliori's Pure Dead series, Ffup the teenage dragon doesn't realize she is female throughout the first book, Pure Dead Magic. In Pure Dead Wicked, she has an, ahem, encounter with what later turns out to be the Loch Ness Monster, and winds up pregnant. Not that she realizes this either, until she goes out to, ahem, answer the call of nature and winds up with an egg ...
- In Harry's Mad by Dick King-Smith, after Mad is bird-napped, Harry's family adopts another African grey parrot to replace him, which they name "Fweddy" due to its lisp. After Mad returns home successfully and gets to know the new parrot, an egg suddenly appears, and Fweddy shyly admits "Call me Fwedwika".
- A poem by Shel Silverstein, "If I Had a Brontosaurus":
If I had a brontosaurus,
I would name him Horace or Morris.
But if suddenly one day he had
A lot of little brontosauri—
I would change his name
- Pit Dragon Chronicles: Jakkin seems to assume that his dragon is male. Most readers are led to believe this too. But the dragon is always referred to as "it" and near the end, Jakkin's master reveals that it's a female.
- Inverted in Safehold. Archbishop Maikel Staynair has a pet cat-lizard that he didn't find out was male until after he'd named it after his late wife. Fortunately, his wife had the kind of sense of humor that the family figures that she would've been amused by it.
- Played straight in Arthur C. Clarke's short story "Who's There?". The protagonist, hearing muffled sounds in his suit halfway through an EVA, begins to fear that he's wearing the same suit that once killed a colleague, who may now be haunting it. Halfway through requesting a check on the suit's history, he is reduced to incoherent screaming panic when something pats him on the back of the neck. Turns out the ship's cat, Tommy, was badly misnamed... and had found a very interesting place to have "his" kittens.
- The dragon in the Discworld book Guards! Guards! is in the end of the story revealed to be female.
- Reversed in the Lensman novels, when the benevolent aliens demonstrate their precognitive powers by telling the hero that he will encounter an immature female creature named "Thomas". At the end of the book, he does meet a kitten whose owner had believed her to be male.
- In Dog with a Blog, the piggy mascot ended up a girl because she had piglets. A rare non-cat/rabbit/guinea pig example.
- Happened in the American Red Dwarf pilot.
- Also in the original Red Dwarf episode "Pete", in which Pete the sparrow is accidentally turned into a tyrannosaurus. At the end of the episode, after they manage to turn Pete back into a sparrow, they come across a dinosaur egg. (A deleted scene elaborates that, yes, Pete is a girl).
- Happened in a Drake & Josh episode where Megan got a pet sheep and named "him" Bob (pronounced like the bleat of a sheep). Soon after, the inevitable happened: Bob had a lamb.
- Leave It to Beaver had a rabbit which the boys assumed was male, but June was able to tell at first glance that it was a pregnant female.
- I know this happened in Takin' Over the Asylum, a BBC Scotland six-episode drama. I can't remember the details exactly, but one of the characters, a mental patient named Francine, vehemently believed the hospital cat was just a fat male until "he" had kittens. (This realization leads to Francine's breakdown, but that's another story).
- Data's pet cat, Spot, on Star Trek: The Next Generation initially was a male cat (and one of a different breed) but was retconned to be female when the plot demanded a pregnancy. One theory is that the original Spot died and Data got a female cat that he gave the same name, which wouldn't be out of character.
- On Sesame Street, after Telly's pet hamster, Chuckie, had babies, Telly renamed the hamster Chuckie Sue.
- One Full House episode had the guys get Michelle a new fish, Freddy, after Martin, the first fish, dies after Michelle gives him a bubble bath. The episode ended with Michelle freaking out after finding a dozen or so black specks in the tank that she didn't put there. When the adults have a look, they realize that they were baby fish. As Jesse remarks afterward, "'Freddy' was a 'Frieda.'"
- Happens with Furys' cat Napoleon on The Revenge Files of Alistair Fury episode "The Luck of the Irish".
- Happened on the penguin episode of Animal Planet's Growing Up _____: a penguin hatchling that was called "he" by its keeper for the first months of its life is ultimately revealed to be female. Blood tests, not pregnancy, revealed the bird's sex in this case.
- Occurred in Steptoe and Son when Harold bought a new horse, "Samson" to replace Hercules who had had to be put down. After Albert delivered "his" foal, Samson became Delilah.
- In one Seinfeld episode, Kramer buys a live chicken in order to get the freshest possible eggs. The next morning, it starts crowing, and Jerry discovers it's actually a rooster. The rooster's name ends up being appropriate anyway:
Kramer: Well, that would explain Little Jerry's low egg count.
- Friends had a similar example: Chandler and Joey's pet chick, who had been referred to as "she" up till then, takes to crowing at sunrise, waking up the gang. As Chandler puts it, "The vet seems to think that’s she’s becoming a rooster... We’re getting a second opinion."
- Phoebe also has a rat named Bob who lives in her apartment. Even when it turns out that Bob is female, Phoebe claims she "likes the name Bob for a girl."
- In The Closer episode "Batter Up", Brenda Lee Johnson has her cat Kitty taken to the vet because of illness. While he's at the vet, "he" gives birth to a litter of kittens. By habit, Brenda continues to call Kitty "he", even after this revelation.
- Gender-reversed in Blackladder the Third. Baldrick has a male cat named Mildred (although that's not surprising what with Baldrick being Baldrick.)
- M*A*S*H has an interesting case in one episode, in which Klinger is trying to breed chinchillas that he bought from a "traveling chinchilla salesman".
Klinger: Now, if you'll just let me cover the cage, Romeo and Juliet are giving each other that look that only lovers share.Winchester: Uh, Romeo and ''Mercutio'' is more like it.
- In another episode, Bugs the rabbit being cared for by a Korean Farm Boy turns out to be pregnant.
- In Jessie, Ravi's pet lizard, Mr Kipling, lays a clutch of eggs; the lizard is renamed "Mrs Kipling".
- In an episode, Timmy rescues an injured chicken he found by the side of the road, nurses it back to health and names it Clementine. Halfway through the episode, the family begins to realize that Clementine is actually a fighting cock.
- The episodes from the 1950s are an inversion as all the dogs to portray the titular Lassie were male dogs. Bonus points in the episode Lassie's Pups were at the end Lassie is shown 'nursing' her newborn puppies.
- Inverted in The Brady Bunch episode "The Hair-Brained Scheme"; the kids were going to breed rabbits, only to find out that 'Romeo and Juliet' were actually 'Romeo and Julius'
- Not surprisingly, this came up regularly on Animal Hospital, a documentary set in a veterinary hospital and hosted by Rolf Harris. The oddest one was a cat called Murphy; the owner originally thought he was a tomcat, but someone else convinced him that 'he' was actually female... and then the vet took a look and discovered he really was male. (He'd been neutered, which, to put this delicately, removes most of the obvious visual cues.)
- During an episode of Degrassi Junior High, two students were studying the effects of junk food on mice. The mouse that ate "healthy" food kept getting fatter and the students couldn't figure out why until the teacher pointed out another reason for a mouse to get fat.
- Inverted in Last Man Standing: Mike just assumes that Muffin is female and only learns otherwise when his neighbor Chuck comes by angry that his female German Shepherd, whom he wanted to breed with another German Shepherd, is already pregnant, claiming that Muffin is the only dog in the neighborhood that could have knocked her up. His initial reaction is to wonder how Muffin would even be able to reach the German Shepherd's genitals, but ultimately he ends up gaining quite a bit of respect for the dog after this.
- Ellen's chameleon Sibyl on Slings and Arrows turns out to be a male. She only learns this by happenstance a year after she gets him.
- "Saturday Morning Confusion" by Bobby Russell includes the following lines:
It's a Saturday morning confusion.
If you think you can sleep, it's illusion,
'cause you'll probably get a rude intrusion from Harry the dog.
Harry the dog is as big as can be
'n' Harry the dog had puppies last week.
We couldn't tell if it's a he or a she, now we know.
- The Gloomsbury episode "The Theory and Practice of Hanky Panky" opens with Vera Sackcloth-Vest and her husband Henry Mickleton discussing how surprised they were that their dog Captain had puppies. In keeping with the genderfluid nature of the household, they continue to refer to Captain by male pronouns throughout.
- In the stage play (and book, and film) I Remember Mama, the youngest daughter's beloved cat, "Elizabeth", is revealed by the girl's brother to be a tom. When the girl presses him on how he found this out, he says "I looked!" before their mother shushes him. And the cat is promptly renamed "Uncle Elizabeth". There are now at least two coffee houses and a women's rock band called Uncle Elizabeth.
- This trope is referenced in "Mister" Mistoffelees's song in the musical Cats.
- In H.H. Munro ("Saki")'s play, The Watched Pot, Hortensia, Lady Bavvel, continually rebukes her servant for referring to her parrot, Adolphus, as "she" — until Adolphus lays an egg.
- In the visual novel Heart de Roommate, Tomoe's fat cat, Toshibo, starts feeling sick. When the heroes take Toshibo to their teacher for help, they learn that "he" is pregnant. The revelation that Toshibo is female was revealed to the player in the previous chapter, however.
- In Pokémon Red and Blue, genders were not displayed for most Pokémon. In Pokémon Gold and Silver - compatible with the first - they were. Woe betide any Pokémon who is given a gender-specific nickname in the first gen and traded to the second. This is particularly bad because the obvious response - changing the poor 'mon's name - isn't available to a Pokémon who has been traded to a different-Generation game (although you can just trade it back to its original game and rename it, though). Certain Pokemon also had this issue on a species wide basis (for example "Mr. Mime" is masculine, though it's Japanese name isn't).
- Also, the Pokémon Azurill has an uneven sex ratio, and is more likely to be female than male. Its evolution, Marill, however, has a fifty-fifty sex ratio, so female Azuril can become male when they evolve. This can be a problem for nicknamed Azurill. This was fixed in Pokémon X and Y; female Azurills remain female after evolution no matter what their hidden stats say.
- In Persona 4, a cool Fox is the Hermit Social Link, who resides at a run-down shrine. Upon completion of the Social Link, the fox turns out to have cubs.
- In Emergency Exit, Fred the cat is a girl. This was discovered by having the neighbor who can talk to cats take care of her for a week.
- In Faux Pas, the two rabbits Stu and Eddie sire an entire rabbit tribe. Apparently the (former) owners didn't notice that Eddie was a doe.
- Happened in Girly, where the cat that the main characters took in got really fat before giving birth, much to their surprise. A bigger surprise is when they find out who's the father, the black cat with the ear splitting cry.
- In Housepets!, one old woman has so many Siamese cats that all look nigh identical that she can't be bothered to name them all. So she names them all Mr. Bigglesworth. Yes, all of them. Not even they can tell each other apart. This results in hilarity.
- In the earliest strips Grape's casual mention of her gender surprises Peanut, who had assumed she was male by default.
- Happened in Sam & Fuzzy, kicking off the chapter Tiny Miracles.
- In the Johnny Wander "Lucky Penny" story arc, Penny has it explained to her that her cat "Boyfriend" is more likely a "Girlfriend" being a calico, with a Wall of Text explanation about the very rare circumstances that might subvert this.
- After eight years of Looking for Group, Cale finally finds out that Sooba is female. At least one of the female characters has known this pretty much all along.
- In the Danny Phantom episode "One of a Kind", Danny finds himself in a literally awkward position when he discovers the gorilla he is researching to raise his grade is actually female. (Sam had earlier mentioned that both known purple-back gorillas are male, meaning that this discovery allows them to at least stave off extinction for one more generation, unless the other one was misidentified too.)
Mr. Lancer: [in class, reading from an article] "Brooding genius, Daniel Fenton did what no other researcher dared to do. He got close enough to this rare purple-back gorilla to realize that Sampson was actually a Delilah." [stops, indignant] Nobody at that zoo ever bothered to see if it was boy or a girl?Danny: [shrugs] I guess they just wanted to respect her privacy.
- In the Doug episode "Doug's Fat Cat", Roger forces Doug to look after Stinky, his Right-Hand Cat. This leads to Doug freaking out when he thinks he's gotten Stinky sick by giving into the "tomcat's" unreasonable desires for pizza and ice cream.
Doug: And now it looks like he's.... he's...
Roger's Mom: A very proud mother.
Roger and Doug: Huh?
Doug: Stinky's a girl? Roger, why didn't you tell me Stinky was a girl?
Roger: Well, how was I supposed to know?
Roger's Mom: (taking him aside) Son, we’re way overdue for a very important talk.
- Subverted in Arthur. The vet announces that the dog has had puppies, and Arthur points out that Pal is a male dog. It turns out the vet was talking to somebody else.
- In Milly, Molly, Brian has a Disney Death ads he is thought to have been eaten by Marmalade the cat. He is temporarily replaced by another mouse, who turns out to be a female named Brioni and they have babies.
- Hey Arnold! has a more justified variation for this in "Das Subway", where a blind man's "male" dog gives birth when they're stuck in a stalled subway. Seeing-eye dogs are typically spayed/neutered but it isn't clear if the dog is a trained Seeing-eye dog or just a pet.
- A 1956 Warner Bros. cartoon, "Mixed Master", had this as the Twist Ending.
- Happened with a stray cat on Recess.
Spinelli: Stuart's a Stella!
- One episode of Bill & Ted's Excellent Adventures featured a rare bird that met its demise (or else escaped) while under the care of our intrepid heroes, causing them to scramble for a replacement. Thanks to their time-travelling phone booth they manage to snag one from its country of origin, with a twist. The original was a male bird. The replacement was a female bird. This is discovered when she lays an egg.
- Rocko's Modern Life had a vet tell Rocko that Spunky is pregnant; however, Rocko knows Spunky is male. In the end, it turns out that the doctor was just confused.
- In one episode of Harvey Birdman, Attorney at Law, Avenger (a male eagle) lays an egg...except, not really. Confusion sets in, and the explanation gets more and more absurd: the egg was actually Potomus' lunch, which he left under the bird in a split second without being seen, Avenger becomes uber maternal over the egg when he decides he really did lay it himself and pokes Potomus in the eye when he goes for it, then it hatches and the chick is a half-bird-half-Jesse Jackson hybrid.
- In one episode of Spongebob Squarepants, SpongeBob adopts a worm whom he names Mr. Wormsley, but then it turns out it's pregnant. SpongeBob spends the rest of the episode trying to find someone to adopt the baby worms.
- One classic Popeye cartoon had his four nephews adopting a horse (rejected "4F" by the Army), and trying to sneak it into the house. When it looks like Popeye has got past them, and the kids are crying because they're going to lose their pet ... Popeye comes back and says, "Well, kids, I guess we'll have to keep her now." Cut to closing shot of family around dinner table ... and somehow there are four baby colts, one for each nephew.
- Subverted in the Phineas and Ferb episode "Perry Lays an Egg". When the boys find the egg in question (which by the way was not actually laid by their pet platypus; it fell out of a nest on the tree and landed by Perry's posterior), Phineas responds: "Perry laid an egg! You know what this means? ...We're gonna have two Perrys!"
- Weird variant from Family Guy in the episode "Mind Over Murder" Peter got into an argument with a coarse-looking, hirsute parent with a husky voice at a ball game, and wound up punching this person in the face. Everyone else was horrified that he'd hit a woman, yet Peter refused to believe his victim wasn't male ... but the punch induced labor and she gave birth right there in the bleachers.
Boy: You hit my mom!
Peter: No, I hit your dad.
Man: Whoa, stand back. Give her some air.
Peter: You mean, "Give him some air".
Woman: Call an ambulance. She's going into labor.
Peter: You mean, "He's going into labor".
- The art book for Avatar: The Last Airbender reveals they once planned to do this with Appa the skybison as part of the ending, after being thought of as the Last of His Kind. The Legend of Korra reveals that Aang found a surviving herd of Sky Bison after the series, averting the problem of Appa being the only male survivor.
- In an episode of Detention, Shareena's pet pig is noted to be fatter than usually and seemed to be sick and one day "he" escaped into the school so they spend most of the episode looking for "him". Near the end they find the pig in the air vent having given birth to a litter of piglets; Shareena admits up to that point she thought Pig was a boy.
- Cartman's cat is named Mr. Kitty. Then came the episode "Cat Orgy". Where it goes into heat... After that, she's just called "Kitty". The FAQ on South Park Studios claims that Kitty is actually a hermophrodite, although this is certainly just something they came up with to explain away any gender inconsistencies rather than something the writers actually take seriously.
- A variant occurs in one Cow and Chicken episode, where the Red Guy steals an egg from a random nest and places it under Chicken while he's asleep. Hilarity Ensues as everybody starts thinking that it was Chicken who laid the egg and that therefore he is actually a she.
- In a story from Charlie and Lola, Lola brought home the class guinea pig, who was called Bert. Lola insisted that Bert was a female guinea pig, even though everyone else insisted it was male. At the end of the story, Bert had guinea piglets. This story was also adapted for a book release.
- One episode of Adventure Time either plays this straight or subverts it: the Ice King brings his main penguin, Gunter, to the hospital, and the two return later with an egg. A shocked Jake yells "Gunter's a woman?!" and the Ice King replies that no, of course he's not...then looks at the area between his legs before shrugging the question aside. Gunter continues to be called male in all future appearances.
- Gorillaz bassist Murdoc Niccals named his raven Cortez and referred to it with male pronouns, but later also claimed for it to lay eggs.
- Camp Lazlo: "Lamar wasn't a Lamar, he was a Lauren!" Noticeably, Lamar is a leech.
- In Dan Vs., Dan gets a cat named Mr. Mumbles, only to be told a few episodes later that the cat is a girl. He doesn't bother to change the name.
- Gender Flipped in one episode of The Simpsons where Bart actually raises a calf in which he names Lulubelle, but then it's revealed that the calf is actually a male, so he renames "him" Lou.
- The Grim Adventures of Billy & Mandy has a sort of weird example with Jeff the Spider, who lays "thousands of thousands" of eggs. Even he thought he was male, and continues to be treated as such afterward, so whether or not this counts as Mr. Seahorse is up for debate.
- An episode of Beavis and Butt-Head has the duo adopting a rat that they find in their house after they try to trap it. The twist at the end is that, after spending the entire episode thinking that the rat was male, they find "him" underneath the grill at the fast food place nursing a litter of newborn baby rats.
- This happens in the episode of The Little Mermaid where Ariel befriends a Bad Luck Creature. Throughout the episode, the characters refer to "Lucky" as "him," but in the end "he" has babies.
- Subverted in the Looney Tunes short "Little Orphan Airedale" (1947): As Porky Pig is trying to throw a freeloading dog out of his apartment, the dog begs him to be gentle because of their "condition" (and gets a sort of maternal look). As Porky relents and gently helps the dog to a chair, he finds out the dog's name is Charlie, and resume throwing him out.
Charlie: Well, there was such a case in Venezuela!
- One know-it-all kid at the zoo in The Penguins of Madagascar spouted off penguin facts, concluding with it being impossible to tell males and females apart without a DNA test. Alice, exasperated, tells him that they have three males and one female, "The birds know which is which." Except they're shocked to find out there's a female. Kowalski does a test that determines Skipper is the female, leading her to seek out Marlene for advice on how to be a girl. In the end, it turns out the test was faulty and all four birds are male, Alice is simply wrong.
- The reverse occurred on Ernest Shackleton's expedition to Antarctica. A tabby was taken aboard the Endurance by a member of the crew and named Mrs. Chippy. A month after setting sail, it was discovered Mrs. Chippy was actually male, but by then the name had stuck.
- In a Growing Up Special on Animal Planet, they had thought that the lion cub they received was a girl. They named the cub Amy; however, as it began to lose its spots they discovered that it was male but didn't change the name.
- It was a kind of tragic discovery really; they had gotten two cubs, a boy and a girl. One sickened and died, and they thought it was the boy (there was a lot going on at the time and there was apparently a mix-up). It wasn't until Amy grew up a little more that they realized which it really was.
- As recounted in The Diary of Anne Frank, Anne was convinced that Peter's cat was female, until Peter flipped the cat over onto his back and proceeded to show Anne the "male organs". Anne's commentary afterward was quite amusing.
- Zookeepers do their best to avoid this trope with captive-bred bird hatchlings or other animals in which sex is not externally apparent. Names aren't assigned to such juveniles until blood tests or the emergence of sex-based traits in adulthood eliminates any doubt.
- The genus name for the beaver is Castor, as it is not possible to tell males from females without... um... very close inspection; thus, the main reason beavers do not normally fall foul of this trope is that few people keep them as pets — however, the capacity for tragicomedy exists.
- Spotted hyena females have genitalia that closely resemble those of males, such that zoos must resort to blood tests to identify their cubs' sexes.
- Due to this, until a few centuries ago, it was popularly believed that hyenas were hermaphrodites.
- A pet actually named George was discovered to be female more than 20 years after receiving the name. In this case, the pet was a turtle, so it really did take a vet to deliver the news. George remained "George."
- Not surprising. One guide to pet turtle book says that unless you are breeding them, selecting a male or a female turtle doesn't matter because they both act exactly the same.
- It takes a bit of practice, but it is possible to sex (tell male from female) a cat by face alone, presuming it was never neutered/spayed. Mature toms typically have prominent jowls while queens have more pointed faces or at least less prominent jowls.
- Guinea pigs can be sexed usually by squeezing gently on the kidney and rump regions. If the urinary hole produces even the tiniest projection, that usually indicates a male. It takes a little practice, or some (possibly Not Safe For Work) photography study on how sexing works.
- In a variation, in 2005 Boston park officials were puzzled by the fact that the swans in the Public Garden, traditionally named Romeo and Juliet, had produced eggs, but they hadn't hatched. Then they found out that both swans were female, meaning that the lack of a pregnancy tipped them off. Given how aggressive swans are known to be, especially in regards to their eggs (to the point that park officials annually have to put up a fence around the nest not to protect the swans, but to protect the dumbass humans who can't resist trying to get a closer look), it's understandable why they were hesitant to check.
- The mascot for Ambrosia Software's office in Rochester, New York, is a foul-mouthed African Grey Parrot named Hector D. Byrd. Then came a trip to the vet where a blood test revealed that Hector was, in fact, female. She has since produced at least two eggs, both unfertilized. Given the persona cultivated by the company regarding Hector, this is probably a good thing...
- In 2009 a Dutch animal shelter got a call to pick up a "tomcat" — a stowaway on a container ship from Norfolk, Virginia that had just docked in Rotterdam. Actually a pregnant female, the cat was featured on a Dutch TV news program for kids, NOS Jeugdjournaal.
- In the early sixties the children's magazine show Blue Peter had a tortoise named Fred. When it turned out to be female, it was renamed 'Freda' - this involved adding an 'A' to the name painted on its shell.
- This happens a lot with parrots as they need a DNA test to determine their sex. Ditto with penguins. This is a problem with birds in general that aren't sexually dimorphic. You need to do a blood test to figure out the sex. Afterwards, they're tagged with metal rings on one leg or the other to distinguish them.
- Dian Fossey — a zoologist famous for observing and protecting mountain gorillas in Volcanoes National Park, Rwanda — described such a discovery in her book Gorillas in the Mist. A gorilla named Puck has been observed from birth for years and was considered a typical young male. Then suddenly a student saw "him" breastfeeding a newborn. The first reaction was "It can't be!" Hence the baby was called Cantsbe.
- Reptiles in general are hard to sex, because it takes knowledge and a gentle probing of the cloaca to figure out their sex, so finding out that a 'male' laid eggs or got eggbound isn't uncommon.
- Fish are another one...many pet fish are identical regardless of gender and quite a few fishkeepers have discovered fry unexpectedly. Can be annoying with fish like guppies that breed fast and fill up the tank. That, and when you DO want to keep the fry, having other fish in the tank means they're probably going to be eaten.
- Sexing chicks for big egg farms takes quite a bit of training and can earn one a substantial wage, in part because of the difficulty of telling males from females, but also because of the difficulty of discarding the unwanted males. And they still miss one every once in a while, as many families that raise chickens can attest. There's even a few brown egg-layer breeds called "sex-linked" that were specifically bred so that cockerels and pullets are colored differently.
- A mature male rabbit has prominent external testicles which are very large compared to the size of his body, especially in smaller breeds. An immature male rabbit who doesn't yet have these characteristics may be accidentally mistaken for a female and vice versa. The tendency of many rabbits to aggressively resist being turned onto their backs only adds to the difficulty. The consequences of this mixup can be quite a handful.
- A summer camp in Canada had adopted a white-tailed fawn and named it Wanda. Then Wanda grew antlers...
- Ask a few friends about their pets and at least one of them will likely have a story similar to this. A possible variation will be two supposed females or two supposed males mating.
- The Phoenix Zoo was embarrassed to admit that for two years they thought a female Andean bear cub was male.
- It is notoriously difficult to differentiate male and female sloths, and there have been a number of cases of zoos receiving sloths of the wrong gender. A pair of sloths show no interest in mating - and the problem turns out to be that they are of the same gender, because one of the pair was incorrectly documented as being of the opposite sex before it came to the zoo.