Some sort of alien lifeform comes to Earth, and, to avoid attention from The Men in Black, disguises itself as a domestic animal, usually a dog. Sometimes the alien happens to already look like a dog, while others may transform themselves using Voluntary Shapeshifting or Applied Phlebotinum.
The latter variant raises some questions as to why the alien wouldn't choose to disguise itself as a human, giving itself far more freedom within human society. However, given that their disguise may not be perfect, or that they may be unfamiliar with human social norms, this may be a sensible decision. A dog that acts or looks a little bit odd is less likely to attract attention than a human who acts strangely or looks creepy.
The alien often finds itself adopted as a pet, usually at least somewhat willingly. After all, a family who is willing to provide food and shelter seems like a reasonable alternative to living on the streets avoiding The Men in Black and dog catchers, and the alien probably reasons that it is capable of escaping if it needs to.
This trope comes in both benign and malevolent variants. The former tends to result in A Boy and His X.
The most common variant of this trope involves a dog that's an alien, however, it can also apply to other types of animals, and "alien" can be substituted for virtually any sentient fictional species. If the alien is not intentionally disguising itself as a dog, but gets mistaken for one anyway, it's Mistaken for Dog.
Compare Alien Animals, where an entire species of Earth animal are and have always secretly been aliens.
- A Budweiser commercial opens with a dog coming out of a house. It runs to the middle of nowhere and gets taken on board an alien ship. The ship takes the dog to the alien leader's quarters on another planet, where the dog presses a button on its collar and reveals that it is actually an alien in disguise. What did he learn on Earth? "WAZZAAAAAP!"
- The comic book story "Rover's a Good Dog!" from Forbidden Worlds #106, which is about a race of evil alien dogs sending an agent out in the guise of a domesticated Earth dog to photograph the plans of a top-secret American weapon. However, the agent becomes compassionate to the inventor and his family, and ultimately doesn't have the heart to go through his mission, instead sacrificing himself to save the family.
- In The Incredible Hercules, we find out that Kirby, the coyote pup that Amadeus Cho had adopted, had been long-replaced by a Skrull imposter. While the real coyote was safe and unharmed, the Skrull coyote had accompanied Cho for the better part of a year before The Reveal.
- Krypto the Superdog is (usually) a dog from Krypton that is passable as an Earth native dog when he's not flying around or showing off his super strength.
- Relative Heroes: Cameron treats his potted plant as a pet/Companion Cube and the thing is eventually revealed to be an Es (a type of shapeshifing alien) advance scout for the now occurring invasion, and so is Cameron.
- Runaways: "My dog's a [mystically cloaked] genetically engineered psychic dinosaur from the 87th century."
- Guardians of the Galaxy: Rocket claims that Captain Marvel's cat Chewie is a rare and dangerous alien species called a flerken. Turns out he's right.
- Stitch from Lilo & Stitch pretends to be a dog despite looking more like a blue koala (even when he hides away the second set of arms, spines, and antenna). According to the tie-in comics, however, it explains that one donor of his genetic composition was from an Earth puppy, so he should have somewhat resembled one. It's not very effective. The universal reaction on seeing him is "You're sure it's a dog?" Basically, he only passes because no-one can come up with any alternatives (and Rule of Funny).
- The Disney Channel Original Movie Can of Worms has the protagonist's new sidekick be an alien talking dog.
- Captain Marvel (2019): The aliens in the movie insist on calling Goose (see above) the tabby cat a "Flerken", and are downright terrified of her, which the heroes find hilarious. And just like in the comics, turns out, the aliens are right.
- Subverted in The Cat from Outer Space, in which the alien looks exactly like a cat in his natural state and doesn't do much to pretend to be one except refrain from speaking. If the humans assume he's a cat, it's not his fault.
- Up to Eleven in Good Boy!, where it turns out every dog on Earth is actually an alien from another planet, though many have gone native and forgotten they were originally sent to conquer Earth.
- In The Hidden, a parasitic alien creature hides in other beings and uses them. It temporarily uses a dog, but (as in the original "Who Goes There?") it wants to be a man, not a dog.
- In I Am Number Four, John/Number Four's dog Bernie Kosar shapeshifts into a more monstrous, distinctly more extraterrestrial creature (that still resembles a dog) known as a Chimera to pull a Big Damn Heroes, engaging another alien monster in combat and eventually killing it.
- Frank the Pug from Men in Black. The Animated Series revealed that under his dog suit he looks like a green dog. Also he is apparently prone to dog-like behavior and it's not part of the disguise.
- In Phantoms, a gelatinous being can send out autonomous drones to do tasks. One of these is a white dog that fools soldiers and gets close enough to send out tentacles and kill them. Later, the dog is used to keep tabs on the surviving humans.
- The alien in The Thing (1982) masquerades as a dog for a while.
- In the short story, "Puppet Show", by Fredric Brown, an old prospector brings a very ugly Rubber-Forehead Alien out of the wilderness on muleback, and the guest is questioned by the military. After a bit of subterfuge, it turns out that the guest is a robot, and the prospector declares himself to be the real alien. The interrogator expresses relief that the master race of the galaxy looks human after all... at which point, the prospector reveals itself to be a robot as well, and the mule asks what a "master race" is and why its appearance should matter.
- Trillian's mice from The Hitchhiker's Guide to the Galaxy turn out to be aliens in disguise. So are all other mice, wild ones included, but Trillian's are specifically implied to be albino laboratory rodents, hence domesticated.
- In Dean Koontz's horror/screwball comedy novel Tick Tock, Del's dog is an alien who has been sent to watch over her. He's very intelligent, but still has the playful personality of a dog.
- In John Scalzi's Agent to the Stars, the gelatinous Blob-like alien the protagonist meets eventually takes on the form of a dog in order to come off as more appealing to humans.
- Inverted in Animorphs, where dogs were apparently created by a race of Ancient Alien androids called the Chee bonding the souls of their dog-like creators, the Pemalites, into wolves.
- Technically, anytime Ax morphs into something other than a human. (Or another alien.)
- The Yeerks once tried to infiltrate "Zone 91" by infesting wild horses, which are allowed to wander on and off the area without people paying much attention. Another time Visser One was working to genetically engineer sharks so that they could be infested and used on Leera, an ocean planet inhabited by amphibious aliens.
- Near the end of the series, the Yeerks steal the morphing cube, meaning that many Controllers can now turn into animals freely.
- In Turning Point, the first book of Lisanne Norman's Sholan Alliance, the Cat Folk Kusac hides his sapience by pretending to be one of the local felines. Carrie then takes him into her home on account of his injuries.
- ALF does this from time to time, with varying degrees of success. For instance, on one occasion, he is seen by a hobo who thought he was a kangaroo... and promptly leaps off the train they were riding.
- Doctor Who: In "The Day of the Doctor", the Tenth Doctor is hunting for a Zygon which, as it turns out, is disguised not as Queen Elizabeth the First, but as the horse that she and the Doctor were riding. Later the trope is subverted, when the Doctor briefly mistakes a mundane rabbit for the Zygon's new disguise.
- Star Trek: The Next Generation: In one episode, the crew of the Enterprise suspect a shapeshifting alien monster to have killed and impersonated a member of a remote science station. The two humanoid suspects (one of them Klingon) are eventually cleared by lab tests, but in a horrifying twist the dog from the station that nobody paid any mind to is revealed to be the alien, and it almost devours Geordi to take his form.
- In Supernatural there are Skinwalkers who can take the form of a dog. One of them becomes attached to the family he lives with.
- In Legacy of the Wizard, Pochi looks like a dog when hanging around the family home, but when selected as a playable character he turns into something that is definitely not a dog.
- Conan the Adventurer: While not an alien, Needle, a baby phoenix, would occasionally pretend to be a parrot to avoid drawing attention.
- The cat in Kid vs. Kat is an alien that looks like one of those hairless cats.
- Invader Zim: GIR disguises himself as Zim's pet dog. ... his disguise being a lame costume with a zipper and a tongue sticking out.
- Futurama's Nibbler is a bit of an odd example as everyone knows he's an alien. What they don't know is that he's a member of an incredibly advanced alien race. And he can talk.
- Counterfeit Cat is a show about the adventures of Gark, a blue tentacled alien who disguises himself as a cat and lives with an ordinary cat named Max.