Alice has just brought home a new puppy! It's adorable at first, but Alice soon starts to notice it behaving strangely. She takes the animal to a vet, or does some research of her own, only to find that her new pet is not a dog at all.
The animal's true identity could be anything from an alien lifeform to a Mexican sewer rat. The most common variant involves the owner mistaking it for a dog, but other pet variants are possible.
See also I Am Not Weasel, All Animals Are Dogs. If another animal is deliberately masquerading as a dog, or has been disguised as one by a human, it may be Ass in a Lion Skin. When the creature is an alien trying to pass as a dog, it's The Dog Is an Alien.
- Played for Laughs in a commercial for Sears Optical: A lady badly in need of glasses lets a raccoon into her house, thinking it's her pet cat.
- Played for Drama in Wolf's Rain, with Quent and Blue's relationship. Quent has a pathological hatred of the wolves (who are fully sentient and capable of speech in the setting), and firmly believes that his hunting animal companion Blue is a particularly large wolf-hound. In reality, she is a half-wolf mutt, but even she believes herself to be a pure-blooded dog and is steadfastly loyal to Quent — at least until neither of them can deny the truth anymore, and they part ways despite considerable anguish.
- The premise of the Lamput episode "Pup Mix" is based around this, with the docs' typical chase to catch Lamput getting them all trapped in an old lady's house. The lady mistakes Lamput and the docs for her dog and treats them as such, going as far as to feed Slim Doc dog food (which he predictably doesn't enjoy).
- In Evolution, someone finds a "dog" which is actually one of the violently aggressive aliens. Hilarity Ensues.
- George of the Jungle: Like in the cartoons, to George, Shep the elephant is his "big, gray, peanut-loving poochie." And Shep seems to believe it too.
- In Adventures in Babysitting, when Brenda has lost her glasses and is stuck in the bus station, she sees what she thinks is a kitten, picks it up... and it turns out to be a jumbo-sized sewer rat.
- In The Pirates! In an Adventure with Scientists!, the pirate's beloved parrot Polly is actually the last living dodo.
- Played completely and horrifyingly straight in The Thing (1982), where the characters bring a malamute home to their base camp and it turns out to be an alien bent on assimilating all life on Earth.
- Experiment 626 (Stitch) of Lilo & Stitch — a genetically engineered creature from an alien planet — is an interesting case. He's mistaken for a dog by the owner of an animal shelter, who really ought to know better. (She also assumes he's dead — having been run over by a semitrailer — and yet puts him in one of the cells anyway.) And that was before he retracted his antennae, back spines, and second pair of arms. Once he has, Lilo is perfectly willing to believe he's a dog. Nani, David, and Mertle's Girl Posse are far more skeptical.
- In Piers Anthony's Race Against Time, one clue that tips off to the protagonist that he's living in a human zoo is that his new "puppy" climbs trees, and his "kitten" has a prehensile tail. Whether they're aliens that only look like house pets, or actual pets modified via genetic engineering, he never does find out.
- In Howard Waldrop's short story The Ugly Chickens, a family has been raising what they think are just very ugly chickens for several hundred years. It turns out that they're actually dodo birds.
- ALF: ALF would sometimes be mistaken for a dog, even getting thrown into the pound in one episode. He was also mistaken for a kangaroo once.
- In Pixelface, at least two sets of visitors to the console (Uthelwynne in "Two Aethelwynnes" and Alexia's parents in "Alexia's Dinner") have assumed Rex is a dog, despite him talking to them.
- The Two Ronnies famous duck sketch in which one of the protagonists has been conned into buying a duck, convinced it's an "Albanian racing pigeon".
- The urban myth of "The Mexican Pet" involves a woman taking her "dog" to the vet, only to be told that it's a diseased Mexican sewer rat.
- There's the old joke that ends, "What the hell kind of dog is that?" (After said "dog" bites off the head of a German shepherd.)
"Well, afore I bobbed his tail and painted him yellow, he was an alligator."
- Happens in a StarCraft cutscene: "Looks like you mashed some poor feller's dog, sarge." "It's a zergling, Lester, smaller type of zerg. They shouldn't be out this far unless... oh shit."
- Tales of Symphonia has Noishe, who Lloyd thinks is just an oversized dog. He's actually Protozoan, a very ancient species.
- In The Tick, the Tick mistakes a capybara for a dog and makes a pet of it.
- Inverted with Ren of The Ren & Stimpy Show; most people think he's some sort of rat, though he's really a chihuahua.
- Stimpy, however, gets passed off as a dog sometimes - when the two of them get jobs at the fire station, and at a dog show. Also, at the show, when Stimpy gets disqualified, their owner George Liquor enters as a dog, and wins!
- In The Looney Tunes Show, Bugs mistakes the Tasmanian Devil for a dog.
- In a series of Looney Tunes cartoons, Sylvester keeps mistaking the baby kangaroo for a giant mouse.
- In the first episode of The Mighty B!, the dog Portia enters in the dog show turns out to be a rat.
- Naturally, this sort of thing happens to Mr. Magoo all the time. One cartoon has him releasing a panther from a circus when he mistakes it for his dog being taken away by the dog catcher. In the feature film 1001 Arabian Nights, Magoo's "dog" Bowser is actually a cat, and a subplot has a ball of enchanted yarn used to make a Flying Carpet take a liking to Magoo after he mistakes it for a cat and starts to pet it.
- Kim Possible: Kim and Ron go undercover at a dog show, entering Rufus (a naked mole rat) as a contestant. Rufus wins!
- The Goode Family: The family adopt what looks like a diseased dog. When they take it to the vet, they discover that it's actually a rare Malaysian marsupial, whose grotesque appearance is perfectly natural.
- On The Grim Adventures of Billy & Mandy, Billy enters a werewolf (not just any werewolf, but an infected Irwin) in a dog show.
- Muppet Babies once had baby Animal mistake a FULLY GROWN DINOSAUR for a puppy.
- On an episode of Family Guy, Peter admits that for the first few years of her life, he thought Meg was a cat.
- On The Angry Beavers, Dag and Norbert are adopted by a family that thinks they're dogs.
- In an episode of Rocko's Modern Life, Rocko tries to get Spunky out of the pound, only for the officials to mistake him for a dog and throw him into a pen with other dogs.
- Another episode had Rocko hypnotized into acting like a dog and ended up being 'adopted' by his neighbor, Mrs. Bighead who didn't recognize him.
- Mr Director would often refer to the Warner siblings as "puppy children". They were really generic cartoon characters with traits from several different animals.
- Runt thinks that his cat friend, Rita, is a dog, and will often correct someone who says otherwise. Since Runt hates cats (Rita being the one exception because he thinks she's a dog), Rita plays along with this.
- Tiny Toon Adventures:
- In the short, "Devil Doggie" from the episode, "The Wheel O' Comedy", Elmyra Duff mistakes Dizzy Devil, a tasmanian devil, for a dog. When Dizzy agrees to be her pet for the day upon hearing the free food she'd give him, she treats him like a dog, which includes playing with a rubber dog toy, feeding him from a dog bowl, and even grooming him to look like a poodle.
- In the short, "K-9 Kitty" from the episode, "Furrball Follies", Furrball is looking for a new home, and a near-blind couple, who absolutely hate cats, mistake him for a dog. Furrball takes advantage of this by acting like a dog. When the couple get two new pairs of glasses at the end of the short, they find out he is a cat and kick him out.
- The Amazing World of Gumball
Richard: It's a labradoodle.
- In "The Puppy", Richard mistakes the vicious turtle he buys from a dodgy van for a puppy.
Richard: It's a labraturtle.
- In "The Burden", Principal Brown asks Gumball and Darwin to look after the school hamster, but Brown accidentally gives them a lump of his hair instead. None of them notice until the next day.
- George of George of the Jungle believes his beloved pet Shep is a dog. Shep is an elephant, though admittedly, he does behave like a dog.
- Truth in television, with sand cats often being mistaken for domestic cats.
- A man found a baby bobcat thinking it was a kitten. At home, he realized his mistake when it opened its mouth and let out a loud baby-like wail.
- In general, pretty much any small wild feline can be mistaken for their domestic counterparts. The worst offenders of mistaken identity are African wildcats and European Wildcats, the latter made worse by frequently interbreeding with feral stock as well being the cats' early ancestors before domestication.