Your Tomcat Is Pregnant

Padre Bateleur: How do you sex such a dangerous animal?
Flinx: In my case, by dumb luck. She had babies.

—Mid-Flinx

So you have a pet and you love him dearly. Let's call him George. One day, you notice that George has become quite fat, you also notice he seems to be sickly. Worriedly you rush George to the veterinarian and ask him worriedly what is wrong with the animal. The vet smiles slyly and informs you that "he" is pregnant. (Apparently you didn't rub George's tummy very often.) Oops.

Of course you still love "him", but now what do you do? Well, you may choose to start calling her Georgette, or you could just stick with George. Especially if she's a dog or some other animal that responds to names. 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 (though the pet getting another animal pregnant could come into play).

Can easily be Truth in Television for rabbits and guinea pigs as well as cats. There are no 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 isn't female. Both situations commonly occur with rabbits and guinea pigs as well as it's hard to identify their genitalia as well.

If this Trope is applied to creatures other than pets, it might overlap with Samus Is a Girl and/or Monster Is a Mommy. If a tomcat is actually pregnant, that's Mister Seahorse.

Examples of Your Tomcat Is Pregnant include:

Advertising

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. This leads to massive Unfortunate Implications.

Anime and Manga

In Aria (the manga, at least), President Maa is normally refered 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 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.
Not a pet, but Ichigo from Bleach mistakenly thought that Yoruichi was a male, based on her (admittedly, arbitrary) masculine voice in animal form.

Comic Books

In the long running Giles comic, the children get Grandma a parrot for Christmas. They name it 'Attila the Hun'. Attila later lays eggs.
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.

Film

The Walt Disney live-action film The Three Lives of Thomasina has this as part of the Backstory, 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.

Kevin the giant bird from Pixar's Up, although the actual pregnant part is skipped over and we just see the chicks.
- 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".
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??!"

Kiara at the very beginning of The Lion King II.

Literature

The book Thomasina (which inspired the above film) involves a female cat mistaken for a male. Her name was originally Tom. The cats gender was revealed with age. Ironically, Tomisina's owner is the daughter of a vet, but he disliked the cat and didn't bother to check the gender for his daughter.
There's a Babysitters Club book where Jessi is petsitting and thinks that a fat old male hamster is sick, only when she takes him to the vet, she finds out that the hamster is pregnant.
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.
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 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' 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
To Laurie

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 "The Haunted Space Suit." The protagonist, hearing muffled sounds in his suit halfway through an EVA, begins to believe the suit is haunted by the ghost of a former colleague who may - or may not - have died while wearing it. Halfway through requesting a check on the suit's history, he is reduced to incoherent screaming panic when something actually pats him on the back of the neck. Turns out the ship's cat, Tommy, is badly misnamed... and had found a very interesting place to have 'his' kittens.

Live-Action TV

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 and 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 when 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 gender 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 Blackadder 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 "travelling 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.

Music

"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
Harry the dog had puppies last week.
We couldn't tell if it's a he or a she, now we know.

Amusingly, the song continues to refer to Harry as "he" all the way to the end.

Theatre

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 promply 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" Mistoffeles' 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.

Video Games

In 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).
  - Not to mention the poor female Mr. Mime...
    - Which is only an issue for games that use the English or French names. All other names for it are gender-neutral.

Web Comics

Happened in the webcomic 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.
Happened in Sam and Fuzzy, kicking off the chapter Tiny Miracles.
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.
- Explained in this strip and here. Also Randy, a male fox, got mistaken by humans for a female fox.
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 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.

Western Animation

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.

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?

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.
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 and 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.
On 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.
On 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!"

Phineas: Congratulations, old boy, you're gonna be a--hey, where's Perry?

Weird variant from Family Guy in the episode "Mind Over Murder" Peter got into an argument with a coarse-looking, hirstute parent 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.
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 Their Kind.
It was made quite clear, in "Appa's Lost Days", that Sky Bison have multiple offspring at a time. If the asexual reproduction thing doesn't work, then Parental Incest would not have been the particular problem encountered ... take from that whatever you will.
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..
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 is a woman?!" and the Ice King replies that no, of course he's not... then checks the relevant body parts before shrugging the question aside.
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!"
Dan of Dan Vs. owns a kitten named Mr. Mumbles, but he doesn't know that Mr. Mumbles is actually a female. This is a bit of a running gag. Even once he finds out, he doesn't change her name.
Inverted 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.
- 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.
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.
There's an old Looney Tunes cartoon where a dog's owner spends most of the story trying to figure out what breed the dog (whom he assumes is male) is; eventually, he has to go on a long business trip, and when he comes back, his wife gives the answer - "he" is a mother, who now has a litter of puppies.

Real Life

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.
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 gender 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.
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."
It takes a bit of practice, but it is possible to sex (tell the gender of) a cat by face alone. 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 whole 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, 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...