Sam: I didn't say it was your fault! And I don't think it's a weight problem. Have you looked at his underside? I think Butcher is a lady... and she's pregnant!
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. note (And 'she' could, of course, be found out by getting another animal pregnant).
This trope heavily overlaps with Lazily Gender-Flipped Name because the pet is already associated with the first name, and changing the suffix is the easiest way even if the resulting name is a bit... obvious. Others might decide to keep the pet's name the same, making it an example of Gender-Blender Name.
Can easily be Truth in Television for rabbits, guinea pigs, reptiles, and cats. There are no coat patterns exclusive to male cats, not even the orange, cream, and apricot ones note , 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 he 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 inexperienced laymen risk hurting the poor snake. If it goes all the way in, it's a boy. If it gets about 1 cm in before you hit something, it's a girl.
- 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 that's not a cow — it's a bull.
- 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.
- Not a pet, but in Bleach, Ichigo mistakes a female shapeshifting character for a man, based on the fact that she has a deep masculine voice when she is in her animal form.
- In a scene in the first episode of 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.
- Inverted in The Life and Times of Scrooge McDuck. One of the stories involves Scrooge bringing two square chickens home to breed. He realized his error when the "female" chicken began to crow.
- Pet Avengers: Although not revealed in-story, this is presumably why May Parker owns a male dog called Ms. Lion. Apparently, she hasn't found out yet, making this an implied inversion.
- Gender-inverted in The Smurfs story "The Egg And The Smurfs", when a Smurf raises up a chick that hatches from the magic egg, thinking that it will grow up to be a hen that will lay magic eggs. To his sad surprise, it grows up to be a rooster.
- Zigzagged in the kids' comic book Some Swell Pup Or Are You Sure You Want a Dog?. An anthropomorphic dog drops off a normal dog (who doesn't get pregnant, she's just a puppy) off at the house of a boy and a girl who seem about seven but live together alone. The kids frequently argue about whether the pup is male or female, sometimes even switching opinions. Eventually, the anthropomorphic dog shouts, "Hey! It's a girl!" which seems to satisfy the boy but the girl still thinks she's a male.
- Ashes of the Past: No actual reproduction involved, but a kid Ash and co. run into asks them for help in figuring out why his Croconaw is upset with him. It turns out she's female... and he'd nicknamed her Steve. Once the kid finds out, she's promptly rechristened as "Terri."
- This parody of Avatar: The Last Airbender has Aang and Katara find out the hard way that Appa actually isn't being overfed.
- In Kitten Companions, a fanfic of Rugrats, Chowder, who's male in canon, turns out to be a female and has kittens.
- My Ridonculous Race: Annie and Leo riding burros through Mexico when one of them starts to go into labor. They assist it in giving birth and by the time they arrive at the chillzone they're covered in various fluids and accompanied by a baby donkey.
- In The SpongeBob Lincoln Power Hour Movie, a crossover fanfic between SpongeBob SquarePants and The Loud House, this is discussed. An egg belonging to a magical creature appears, and Leni thinks it was laid by their (male) canary Walt and he's "secretly a girl".
- How to Train Your Dragon: Fishlegs' dragon, a Gronckle named "Meatlug," was thought to be a boy until Gift of the Night Fury, where Meatlug is revealed to have laid a bunch of eggs, which causes Ruffnut to tell him, "Your boy dragon is a girl dragon!" While Fishlegs does seem a little surprised, he ultimately responds with, "That would explain a few things."
- An implied inversion occurs in Ice Age: Dawn of the Dinosaurs: Sid tries to milk a large mammal to get food for three dinosaur babies, but it chases him and he doesn't get milk. Sid exclaims, "I thought you were a female!" so it might have been male, but it might have just been angry and not lactating.
- 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, with their son Kion only coming along much later in the The Lion Guard TV series. 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.
- 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 again. Despite the interest the plot thread generated, it was dropped from the film for being "too bizarre".
- Alpha (2018) is about a prehistoric teenager who befriends a wolf he dubs "Alpha". Near the end of the film it's revealed that Alpha is female and that she is about to have a litter.
- Defied in Caged. A kitten is named a Gender-Blender Name "Fluff" just in case it's either male or female.
- 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.
- This is the main twist in Godzilla (1998).
Nick: We know he eats a lot of fish, he's amphibious, he's a burrower and...he's pregnant.
- Inverted in I Remember Mama, where one of the girls has a cat named "Elizabeth". Even when told that he is a tom, she refuses to change it.
"And I've been undressing in front of you for all these years??!"
- In The Million Dollar Duck, Jimmy assumes his pet duck is male and names it Charley, until it starts laying eggs.
- 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!
- 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 Uncanny, Valentine discovers that Scat is female when she gives birth to kittens. He had been referring to Scat as "he" up to this point.
- Animal Inn (by Virginia Vail): In book 1, Val meets Toby Curran when he brings Harvey, his little brother Jake's rabbit, to Animal Inn, thinking he's sick - he hasn't been eating right (but is looking pretty fat), and has started pulling out his fur. Val, hearing the symptoms, deduces that Harvey is in fact a pregnant doe, something her father confirms, and the rabbit gives birth to a litter of six late in the book.
- 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.
- The Robert Westall book Blitzcat stars a female cat named "Lord Gort". It's explained that they thought she was a male, but by the time they find out the truth, she was used to answer to that name, so they kept it.
- In the Captain Underpants series, George and Harold's pet pterodactyl, Crackers, is initially thought to be a boy. However, things start to reveal themselves when the 3D Hypno Ring, which is notorious for having inverse effects on women, fails to affect Crackers, and in the 10th book, Crackers is revealed to have laid three eggs, confirming to George and Harold that she's a girl. Amusingly enough, the hatchlings turn out to be the children of Crackers and the other pet of George and Harold, Sulu the bionic hamster, much to Harold's disgust.
- When Holden in The Catcher in the Rye goes to see a movie, the movie ends with everybody laughing when their great Dane comes in with a litter of puppies.
Holden: Everybody thought it was a male, I suppose, or some goddamn thing.
- In Clan Ground, Thakur adopts a lemur-like "treeling" and thinks it male until "he" disappears and comes back pregnant.
- In Clementine and the Family Meeting, Clementine and her whole science class are doing projects with rats. She is partnered with Waylon and they have a rat named 18 whom they've been feeding snacks as a test to whether it will help him learn better. He grows bigger and bigger and they think it's the snacks causing it. Later, he goes missing and when they find him, they find that "he" has had babies and is actually a mother rat.
- Inverted in Delilah Darling Is In The Classroom. Delilah babysits the hamsters Polly and Dolly, but they have babies as it turns out Polly was actually a Wally and Wally and Dolly have eleven babies.
- Inverted in Dewey: The Small-Town Library Cat Who Touched the World. Author Vicki Myron recounts, in the epilogue, how she ended up with another kitten a few years after Dewey died, whom she assumed to be female and named Page. At the very end, when she's taken Page to the vet for her first checkup, the doctor tells her a few issues that Page does not have, then says "I have, um... news." Vicki, recalling this is the same room where Dewey died, fears the worst until the doctor smiles and says: "Page is a boy."
- In Diary of a Wimpy Kid: Hard Luck, Greg's aunt Gretchen's rabbit is thought to be a buck, but she has babies.
- The dragon in the Discworld book Guards! Guards! is in the end of the story revealed to be female.
- 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.
- 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.
- In one of the Judy Moody books, Jessica Finch mentions her guinea pig, Chester, who turned out to be a girl and had four babies.
Jessica held up a picture. "Nutmeg, Jasmine, Coco, and Cindy, short for Cinnamon. The Spice Girls!"
- 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.
- Harry's Mad: 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, "Fweddy" surprises them by laying an egg, and shyly admits "Call me Fwedwika".
- A poem by Shel Silverstein from Where the Sidewalk Ends, "If I Had a Brontosaurus", is about a hypothetical pet brontosaurus being discovered to be female when it has babies.
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
- In the Lensman novels, 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 the picture book Little Grunt and the Big Egg, a caveboy finds a dinosaur and names it George. Later, "George" lays eggs so the caveboy renames it Georgina.
- In the children's book Mr Cat, a man's cat named "Mr Cat" is revealed to be female when she has kittens, possibly with the crazy grey cat next door. However, she is still called "Mister".
- In The Mystery of Mr. E, a book from the WellieWishers line of the American Girls Collection, this occurs with the Welliewishers due to an Accidental Misnaming. The twist in this case is that the dog's owner actually already knows she's a female dog. Willa's Aunt Miranda asks the Wellwishers to look after "Mr. E," a dog that belongs to her friend Shu Ping, while he travels in China. The girls agree, but one day, the dog disappears. The situation is finally resolved when they discover Mr. E snug in a little cave with several puppies. They take Aunt Miranda to see and finally it occurs to Willa to ask how "Mr. E" could have had puppies if he's a boy. Aunt Miranda asks if she said "Mr. E" and explains that the dog's name is actually "Mystery". She had mysteriously appeared at Shu Ping's house one day and he could never find her owner, so he kept her and named her "Mystery".
- Bill (later Billina) the chicken from Ozma of Oz and later Oz stories.
- 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.
- Percy Jackson and the Olympians: Inverted with the Ophiotaurus, whom Percy starts calling "Bessie", a traditional name for a female cow, when he first meets it. Later, when the animal shows up after Percy got information from Nereus, Grover (who's meeting it for the first time) is able to translate its speech and identifies it as a male. Percy still refers to the animal as "Bessie" out of habit later, surprising even Poseidon.
- 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.
- 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 ...
- 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.)
- 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.
- In The Six Bullerby Children, one of the boys finds a chick and names it Albert, but it grows up and turns out to be a hen, so he renames her Albertina.
- 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, the trope is inverted when she names three of the kittens Molly, Holly and Polly, but it turns out they are toms, and "Polly" gets renamed Ollie. note
- Inverted in Stray (1987). After Pufftail gets thrown out of a moving car and rescued by nuns, a few nuns mistake him for a female. They call him "Mildred", which confuses him because he doesn't see a female cat in the room. Eventually another sister realizes he's male and begins calling him "Miles".
- 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.
- 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.
- The Unadulterated Cat proposes the theory that all the headlines in UK newspapers about cats found in lorries refer to the same cat, who is trying to get somewhere, and describes this cat as "a small black-and-white tom". They acknowledge that headlines about a cat having kittens in a lorry reveals a slight flaw in the theory.
- 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.
- Taylor from Wild Orchid thinks her gerbil Harold Pinter is a male until he gives birth to a litter of seven. Even after that, she keeps calling him by male pronouns.
- In the Young Amelia Bedelia book, "Amelia Bedelia Gets a Break", Amelia Bedelia cares for her class's pet hamster, Harry, during a school break. Harry escapes from his cage and takes up residence in Amelia Bedelia's dollhouse. Amelia Bedelia and her classmates find the hamster there with six babies and rename her Harriet.
- 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.)
- 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.
- The Army Game: In "Snudge's Budgie", the boys from Hut 29 attempt to replace Snudge's pet budgerigar with a lookalike after Bootsie lets the original one escape. They might have got away with it if their replacement 'male' budgie hadn't laid an egg.
- Gender-inverted in Blackadder the Third. Baldrick has a male cat named Mildred (although that's not surprising what with Baldrick being Baldrick.)
- 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'.
- Cheers: One episode has Carla getting a cat. She never cared about what sex it is, but the cat gives birth to kittens when showing it off to the bar. Carla moans she swears the cat wasn't pregnant when she got it, but the second it became a Tortelli, it became pregnant and started dropping kittens.
- 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.
- Death in Paradise: In "Wish You Weren't Here", DI Mooney is looking after a pet mongoose/polecat/ferret thing named Sherman until an acquaintance can persuade his wife to allow it in the house. The animal escapes and Mooney buys a replacement. While Mooney is at the bar, Ruby examines Sherman Mk. II and remarks to JP that DI Mooney might want to hold off on giving this Sherman to Darnell. When JP asks why, she says because Sherman Mk. II is a girl.
- 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.
- 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 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.
- In EastEnders, Jim Branning accidentally killed his wife Dot's male budgie and had to replace him. When she returned from holiday, she had a new male budgie to act as a friend. Jim ends up confessing after the budgies produce an egg.
- This show 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 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."
- 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.'"
- An off-screen example from Hannah Montana: In one episode (from when the characters were still in middle school), Miley tells Lily that when she and her family were still living in Tennessee, they once had a pet pig named "Luanne," who the Stewarts initially thought was a boy (having originally named the pig "Lou") until said pig gave birth to a litter of piglets.
- In the It's Always Sunny in Philadelphia episode "A Woman's Right to Chop", Mac takes his dog Poppins to the vet for the first time in ages and the veterinarian tells him Poppins is pregnant. Mac doesn't understand how a male dog can get pregnant and the veterinarian has to try to explain to him that Poppins is female. Mac leaves with the idea that Poppins is a pregnant male dog. It's actually a different dog that looks like Poppins.
- 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 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.
- Leave It to Beaver had a rabbit which the boys assumed was male, but June was able to tell at first glance that she was a female (and pregnant).
- 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.
- An interesting case in one episode, in which Klinger is trying to breed chinchillas that he bought from a "traveling chinchilla salesman".
- On My Name Is Earl, Earl had gerbils as pets, and thinking they were both males, named them "Lenny" and "Squiggy." The gerbils did not go with Earl when he left home (or, more precisely, was kicked out by his dad). Soon enough, they started reproducing like crazy, several generations of gerbils crammed into one cage, much to Earl's surprise when he attempted to move back in with his parents after finding out that Joy cheated on him.
Randy: Lenny and Squiggy... [Beat] ...Did you know they were gay gerbils when you named 'em?
- In Ninja Turtles: The Next Mutation, the Turtles find a non-mutated turtle wandering through the sewers. Mikey initially refers to it as male before Venus corrects him saying it's female. He then asks her how she can tell. She just replies that he and Splinter need to have The Talk.
- The Partridge Family: In "Tale of Two Hamsters," Danny buys a hamster of either sex in the hopes that he can make money breeding them. He names them Dean Martin and Sonia. Naturally, Dean Martin ends up giving birth.
- Parks and Recreation: In the Season 2 premiere "Pawnee Zoo," Leslie creates an accidental scandal by conducting a marriage ceremony for two of the Zoo's penguins for a publicity event, not knowing both were male. The boys at The Bulge honor her when she refuses moralists' calls to annul the penguin betrothal.
- 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).
- Happens with Fury's cat Napoleon on The Revenge Files Of Alistair Fury episode "The Luck of the Irish".
- 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.
- Ellen's chameleon Sibyl on Slings & Arrows turns out to be a male. She only learns this by happenstance a year after she gets him.
- 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. It must be noted that by the next episode, Spot's kittens were gone and Spot kept being referred to as 'him'.
- In the Star Trek: The Original Series episode "The Trouble With Tribbles", Uhura finds a little alien called a Tribble. She wants to keep it as a pet but thinks it's a male, but then it has "babies". It turns out that Tribbles are all born "pregnant".
- 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.
- This happened in Takin' Over the Asylum, a BBC Scotland six-episode drama. 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).
- The Worst Witch: The 2017 series has Maud find out Midnight is really a female after she has a litter.
- "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.
- On Sesame Street, after Telly's pet hamster, Chuckie, had babies, Telly renamed the hamster Chuckie Sue.
- 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 Men from the Ministry episode "A Sense of Power" Lamb's new pet hamster Harry is acting restless, and he decides to buy another one called Charlie for company before leaving to Scotland for negotiations with the locals. When he and Lennox-Brown return it turns out that Charlie has had a litter.
Lamb: You mean he's not a Charlie, she's a Charlotte?Lennox-Brown: Obviously the only Charlie 'round here is you!
- In Into the Woods, Jack refers to his cow, Milky White, as a male even though his mother keeps telling him that Milky White is female, since only females can produce milk. Of course, Jack isn't terribly bright.
- 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.
- 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 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 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 (while you can trade it back to the Gen I game and rename it, that ceases to be an option if you're on the Virtual Console and transfer it into the later games). Certain Pokemon also had this issue on a species wide basis (for example "Mr. Mime" is masculine, though its 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.
- Shantae: Half-Genie Hero: After Mermaid Falls, and as part of a Chain of Deals starting with getting Salted Caramel, a pet gets a name change:
I guess that you're actually MRS.McBeaklin after all!
- According to modeler Druelboso, the Light Music Club's pet turtle Mr. Jazzy Feet in Yandere Simulator is actually female, and was named by high school girls who didn't check her gender.
- Summed up in this Chicken Thoughts strip. A person assumes a parakeet is male just by pure guesswork (since it's hard to sex a parakeet) and names her Bob, but she turns out to be female and lays an egg.
- 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.
- Faux Pas:
- 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.
- 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.
- 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.
- Happened in Sam & Fuzzy, kicking off the chapter Tiny Miracles.
- 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. To make things weirder, a kitten hatches from the egg. This may be justified in later seasons when we learn Gunter is actually the contained form of an Eldritch Abomination whose real name is Orgalorg.
- ALVINNN!!! and the Chipmunks has Mr. Whiskers, who was revealed to be a female at the end of the episode "Good Luck Mr. Whiskers", and being renamed to Miss Whiskers.
- Angela Anaconda has a terrier mix named King. It's revealed in passing that "King" is really a "Queen," so presumably this trope came into play at some point.
- 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.
- 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.
- 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.
- The Berenstain Bears: In "The Trouble With the Pets", the cubs get a puppy and think of naming it Prince, but are informed it's a female, so they name her Little Lady. When she grows up to be quite a large dog, they rename her Lady.
- 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.
- Camp Lazlo: "Lamar wasn't a Lamar, he was a Lauren!" Noticeably, Lamar is a leech.
- 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 she was male. At the end of the story, Bert had guinea piglets. This story was also adapted for a book release.
- The C.O.P.S. (1988) episode "The Case of the Kangaroo Caper" featured a boxing kangaroo named Bruce, who turned out to be female when Rock Krusher saw that she had a joey in her pouch.
- Cow and Chicken:
- A variant occurs in the episode "Which Came First?", 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 "Me An' My Dog", Cow starts playing with an imaginary dog named Kevin, but she loses him and goes out to search for him. When she returns she tells them that she gave him to a sad and lonely imaginary man who needed Kevin more than she did... and then the bell rings. It's the imaginary man, who informs everyone that Kevin gave birth and wanted to show the puppies to the family... and then it's bizarrely subverted as the imaginary man says that the weirdest part is that Kevin is a boy.
- 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.
- 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.
- DC Super Hero Girls: Kara has to babysit Z's pet rabbits. Kara assumes they're both male and dubs them Merlin and Harry Houdini. After turning her back for a few seconds and finding some new babies, Kara proudly declares Harry Houdini a mother. Of course, Z knew their real genders and had specifically told Kara to keep them separated to avoid this, but Kara hadn't paid attention to Z's instructions.
- In an episode of Detention, Shareena's pet pig is noted to be fatter than usual 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 that up to that point she thought Pig was a boy.
- In Detentionaire, Biffy adopts a kitten and names them Mr. Rumplekittycat. A few episodes later, he reveals to Lee that the cat's actually a Mrs., and that he's planning on knitting sweaters for her (never-seen) kittens.
- 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...he's....gonna...
Mrs. Klotz: Be a very proud mother.
Roger and Doug: Huh?
[a nurse appears with Stinky in a box with newborn kittens]
Doug: Stinky is a girl? Roger, why didn't you tell me?
Roger: Well, how was I supposed to know?
Mrs. Klotz: (taking him aside) Son, we’re way past due for a very important talk.
- Dr. Dorsal, the vet, even says that feeding Stinky pizza and ice cream was "just what nature ordered", though in real life, this would be terrible advice, as cats are lactose intolerant and often get diarrhea from dairy products, and garlic is downright poisonous to cats.
- Dragons: Riders of Berk: In one episode the dragons mysteriously leave for a period, confusing the Vikings until they find a clutch of eggs laid by Meatlug, much to Fishlegs' surprise as he'd assumed dragon-mount was a boy, but then realized that Meatlug being a girl "would explain a few things."note
- The Fairly OddParents! episode "Hare Raiser" ends with the revelation that the class pet Mr. Cuddles is actually a girl rabbit and has babies.
- 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: Hey, 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".
- Famous Fred: Fred was initially thought to be female when he was first adopted, and almost got named Brenda, but then his owner quickly checked the sex and renamed him Fred.
- Gorillaz bassist Murdoc Niccals named his raven Cortez and referred to it with male pronouns, but later also claimed for it to lay eggs.
- 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 Mister Seahorse is up for debate.
- 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.
- 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.
- In the HouseBroken episode "Who's a Good Therapist?", Chico, who until this point was presented as a male overweight tomcat, is revealed to be pregnant as they start giving birth in the middle of a session. It comes as a big surprise to everyone, even Chico.
- This happens in the episode of The Little Mermaid (1992) where Ariel befriends a Bad Luck Creature. Throughout the episode, the characters refer to "Lucky" as "him," but in the end "he" has babies.
- Looney Tunes:
- Subverted in the 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 actually was such a case in Venezuela!
- In the 1956 Warner Bros. short, "Mixed Master", a man named Harry brings home a new dog, Robert, but his wife, Alice protests, since the dog appears to be a mutt, and they already have Chang, a purebred Pekinese. Between not being able to figure out what Robert is, and the fact that he seems to clash with Chang, Alice is ready to get rid of him, but Harry insist that they keep him. In a Twist Ending, Harry comes back home from a business trip and Alice reveals that she found out what Robert really is. Cue Robert coming in with a litter of puppies.
Alice: "Robert" is a mother. Aren't you, Roberta?
[Chang gives the audience a knowing smile]
- Subverted in the 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.
- The Loud House: In "Along Came a Sister", Lincoln offers to take his 5th-grade class's pet tarantula, Frank, home for the weekend—everybody (except for Lisa Loud) thought Frank was a boy, but in the end, Lisa reveals that Frank's actually a girl, explaining that she was able to tell the spider's sex due to some markings on her back ("Frank" is ultimately renamed "Frances"). Also at the end of the episode, a spiders' nest is seen, which is implied to be Frances's because Lisa notes that tarantulas are always sluggish (which Frances had been) before giving birth.
- Martha Speaks: While Martha didn't get pregnant, she does tell a story of a time when a woman said "Here, boy" to her and she thought "Hello, I'm a girl dog".
- Milly, Molly:
- Brian the mouse has a Disney Death as he is thought to have been eaten by Marmalade the cat. He is temporarily replaced by another mouse, who is thought to be "another Brian" but turns out to be a female named Brioni and they have babies.
- Zigzagged in the episode "Beaky". Milly and her family look after a duck named "Beaky" and call Beaky a "he", and then when they release Beaky in the park, s/he and his/her friend (who later became a mate) have had ducklings. The humans assume that Beaky's the mother and they mistook the sex, however, s/he could be the father.
- 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.
- 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!"
- 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.
- Happened with a stray cat on Recess—the cat was initially assumed to be a male (who the kids named "Stuart"), but after the cat gave birth to a litter of kittens, she was renamed "Stella."
Spinelli: Stuart's a Stella!
- 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.
- Gender-Inverted Trope in one episode of The Simpsons where Bart raises a calf he names Lulubelle, but then it's revealed that the calf is actually a male, so he renames "him" Lou.
- South Park: 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 hermaphrodite, although this is certainly just something they came up with to explain away any gender inconsistencies rather than something the writers actually take seriously.
- SpongeBob SquarePants:
- In one episode SpongeBob adopts a worm whom he names Mr. Wormsley, but then it turns out she's pregnant. SpongeBob spends the rest of the episode trying to find someone to adopt the baby worms.
- In the episode "Spot Returns", after noticing that Spot has gotten fat, Plankton and Karen find out through X-Ray that Spot is pregnant, not fat- much to Plankton's surprise since he assumed Spot was a boy, until Karen reveals that amoebas can reproduce asexually. The entire plot of the episode is about putting Spot's amoeba puppies under control.
- Drutt from The Trap Door is referred to as 'he' throughout the series but gave birth to four baby spiders towards the end of the show.
- 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, a cat that had taken up residence was assumed to be female because "her" stomach began to swell, but as time progressed and no kittens appeared, they realized the cat had just gotten fat. It's Peter who determines the cat is male, which he proceeds to prove to Anne by flipping the cat over onto his back and showing 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 (in the Middle Ages, it was thought males deterred predators by ripping off their genitals and throwing them); 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 as 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.
- Similarly to the above, it can be difficult for a layperson to determine a tortoise hatchling's sex by just looking at it. And, unlike turtles, tortoises' behavior does change depending on their sex—they aren't social animals in general, but males will be particularly aggressive towards one another. Luckily, it gets easier to distinguish males from females as the tortoises grow older.
- 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. Some male cats have thin faces too despite being intact, so this is not a fully 100% accurate way to tell (even with practice), but you'll still be correct more often than not.
- 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 Cantsbee.
- 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. Also, some species can change gender in a single gender environment. Clownfish for example.
- 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 sex. A pair of sloths show no interest in mating - and the problem turns out to be that they are of the same sex, because one of the pair was incorrectly documented as being of the opposite sex before it came to the zoo.
- Budgies can have this issue. It's usually easy to sex adults - males have blue ceres while females have pink or peach ceres - but this changes depending on the mutation. Depending on their coloring, some males might keep their light ceres even into adulthood. Juvenile budgies also have similar ceres no matter their sex, which can cause people to get their sex wrong.
- In November 1928, a hippopotamus left its waterhole in the St. Lucia Estuary in Zululand and set off on the 1,600 kilometres (1,000 mi) journey to the Eastern Cape, finally arriving in East London in March 1931. The animal, thought to be male and named Hubert by the press, became a minor celebrity in South Africa and attracted crowds wherever they went. A month after arriving, Hubert was shot and killed by farmers (despite having been declared royal game, and thus protected, by the Natal Provincial Council); examination of the body revealed that Hubert was actually female, and posthumously renamed Huberta; her taxidermied body was placed on display at the Kaffrarian Museum (renamed the Amathole Museum in 1999).
- Mature male betta fish have prominent, colorful fins; females and immature males don't. So it's not uncommon for an immature male to be mistakenly sold as a female. (This can present a problem if he was placed in a "sorority tank" with females, or in a community tank with fish such as mollies.) The best way to tell is to look for a white tube-like projection under the fish's pectoral fins. This is an ovipositor, and if one is present, it's a female.
- One common cat the inverse happens with, minus the pregnancy, is calico and tortoiseshell cats. Most are female because it’s a recessive sex-linked trait. Occasionally someone gets surprised to find they have a male Kleinfelter’s, aka XXY, or in more rare cases, a chimera (two sets of DNA) male calico. Most XXY males are sterile, though.
- Mark Rober found out the fat, gluttonous squirrel he nicknamed Fat Gus was not only a girl but also pregnant at the time. Yes, he did accidently call a pregnant squirrel fat.
- In 2023, an 8-year-old gorilla in Columbus Zoo, assumed to be a young male, unexpectedly gave birth to a baby. It is important to note that gorillas have inconspicuous external genitalia and a rather large abdomen, which explains how the pregnancy went unnoticed.
- A black bear dubbed “Hank the Tank” became internet famous for breaking into houses in Southern California and “his”, er… chunkiness. Only to be revealed as a female with three cubs when they were captured in August 2023.
- Many pet store chains try to minimize the risk of small mammals being sold pregnant by dealing solely in one sex of rodent at each of their outlets. Even so, plenty of new pet owners who'd thought their new cage pet was male have found themselves saddled with unexpected "oops" litters, due to vendors' errors in sexing and segregating their juvenile mice, gerbils, guinea pigs, etc. The opposite is also known to happen, as pet owners with what they thought was a female pet grew concerned about those mysterious lumps that developed as the animal got older.