A popular trope in comedies and a subtrope of Hilarity In Zoos. An animal escapes from a zoo, a circus, a pound, a laboratory or some other kind of area where it formally lived in restricted conditions. The creature might be dangerous, like a Killer Gorilla, a lion, a bear, a tiger, a wolf, a rhinoceros or an elephant, or just something that might provide zany antics, like monkeys, a kangaroo, a seal, ...
Invariably our protagonists will always encounter this escaped animal. Either they have to fight it off and bring it back to the zoo in order for a huge award, or they help it remain hidden and even might transport it back to its natural environment.
A trope Truth in Television, as several animals have been known to escape and create panic and chaos along the way. In Real Life this is usually not that funny, as people might get injured or even killed. And not only with obviously-dangerous animals. Innocent looking creatures, like a chimpanzee, can get very aggressive if they feel threatened. The animals themselves may even get shot by zookeepers and/or the police.
Also note that this trope is exclusively meant for animals that were held captive by humans, not animals entering the human world from a forest or something.
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BIONICLE: After the Great Cataclysm, the Rahi stored in the Onu-Metru Archives escaped and roamed the island of Metru Nui, exacerbating its ruined state.
Anime and Manga
The second Cowboy Bebop episode, "Stray Dog Strut", has the protagonists hired to hunt down a dog escaped from a lab, or more specifically, the criminal who tried to steal it. Meanwhile, the lab workers try to recapture the dog with a giant dog whistle, which results in a riot of stray dogs running through the streets. The protagonists end up with the dog, never knowing it's been genetically engineered to be fantastically intelligent and is worth a fortune.
The anime Sword For Truth begins with a giant tiger escaping from its cage and slaughtering a large number of the Shogun's men before being cut down in one stroke by protagonist Shuranosuke Sakaki.
Donald Duck. One Carl Barks comic strip had Donald Duck walking around with a necklace that brings good luck. When he hears that a dangerous gorilla escaped he decides to turn the giant ape in, believing that he will be prevented by his Lucky device. Of course, it doesn't work.
In "De Straatridder" ("The Street Knight") Lambik helps several zoo animals escape and hides them in Suske en Wiske's home.
In "De Apekermis" ("The Monkey Fair") the apes in the local zoo become intelligent and decide to escape to take over the world. note This story was drawn way before the Planet of the Apes movies were made, though it may have been inspired by Pierre Boulle's novel of the same name. .
In Hannibal a baby elephant escapes from the circus and is adopted by little Clo-Clo, who keeps him as a pet. The animal is stolen back to the circus and Clo-Clo is kept imprisoned there too. In the end Nero and his friends liberate Clo-Clo, but Hannibal suddenly disappears. This leaves Clo-Clo depressed and he waits and waits for his animal friend to come back. In the end Hannibal does come back, but he is an adult sized elephant now! The Happy Ending shows the others still welcoming him back in their midst.
Subverted in Allemaal Beestjes where Nero is transformed into a lion and then caught by frightened people who bring him to the zoo, despite nothing having done anything. A similar storyline can be found in Zwoele Charlotte, where he is dressed in a gorilla costume and again sedated and brought to the zoo, because people think he is a real ape.
De Kiekeboes. Fanny saves a chimpanzee from a medical laboratory and keeps at her house. The animal then escapes from her room and goes to the neighbours where he climbs inside the bathroom and helps the unknowing woman of the house with scrubbing her back.
Pride of Baghdad by Brian K. Vaughan, in which a pride of lions escape the Baghdad Zoo during the U.S. invasion.
Spirou and Fantasio. In the story "Le Voyageur du Mésozoïque" a dinosaur escapes from the Count of Rommelgem's laboratory and causes havoc in the city.
Mortadelo y Filemón. In the album "Pànico en el zoo" all zoo animals escape and Mortadelo and Filemon have to catch them.
The Discworld fanfic Nature Studies looks at various animal escapes in Ankh-Morpork. It starts in canon with the escalating problem with the Royal Mail and its mailboxes. (as described in Pratchett's novel Making Money). Snails were eating the mail. Frogs and toads were inserted to eat the snails. The frogs bred. Snakes were inserted to eat the frogs. Then mongeese needed to go into the boxes to deal with the snakes - one of whom turns out to be a Howondalandian Spitting Mamba and not a harmless Lancre Grass Snake. This difficulty is dealt with by experienced hands. And then CMOT Dibbler takes it on himself to bring the joys of exotic wildlife to the city in the form of a Safari Park. A Dibbler disaster ensues and Howondaland's native animals escape...
Laurel and Hardy. In The Chimp a huge ape escapes from the zoo and frightens Laurel and Hardy.
King Kong: After Kong arrives in New York City he is exhibited in a theater. When he escapes he terrorizes the city and... well, you know the rest.
Jurassic Park. John Hammond creates a zoo for dinosaurs. He plans it as a tourist attraction, but when a storm causes the electric power to fall out the monstrous animals are able to come from behind the fences and have the humans run for their lives.
The second installment The Lost World has a Tyrannosaurus Rex taken to the continent with the intention of putting it in a zoo, but he escapes before they could do so.
Jumanji: In this film a magical game board causes all kinds of animals to break loose from the game and rampage through town.
Happened in a a Christian direct-to-video movie about a talking puppy named Spunky in the episode "Circus Adventure", where the circus animals break loose.
A funny subversion occurs in the 1968 version of Planet of the Apes, where Taylor the human escapes from a medical lab where apes do experiments on humans and run amok, scaring the innocent residents of Ape City.
Rise of the Planet of the Apes: Apes escape from their own facility and release other apes from a zoo to create an army, before disappearing into the forests.
Zebra in the Kitchen: A 1964 movie in which somebody lets all the zoo animals go free.
In Wild Beasts, PCP on Frankfurt's water system causes animals to go crazy, and various dangerous beasts escape from the local zoo to terrorize people.
During the climax of Gangs of New York an elephant escapes the Barnum and Bailey circus during the Draft Riots, though the city is so overcome by chaos the elephant rampaging down the street is only another small part of the madness the city has descended into.
The final act of Twelve Monkeys shows the titular Animal Wrongs Group releasing the animals of the local zoo, causing all sorts of pandemonium and their "we did it!" message spray-painted all over the city setting them up as the Red Herring for the cause of The End of the World as We Know It.
In the second The Naked Gun movie, Frank accidentally causes a break out at the zoo, and animals are seen running around in the background of some scenes. In the end, the Big Bad is dispatched by an escaped lion.
The first of The Green Hornet Serials: one episode dealt with the gangsters trying to take over a zoo. We see a tiger get loose due to sabotage, and it's indicated that this wasn't the first time.
Good Burger. The reason Heather was committed to Demented Hills was because, "Small space aliens landed in her head and told her to break into the zoo and free all the kangaroos."
Animal Farm. Farm animals break loose and - unprecedented in the animal world - take over the place and establish a revolutionary community.
Curious George. A chimpanzee who escapes from a zoo and has wacky mischievous adventures.
In the novel version of The Man with the Golden Gun, Scaramanga killed his first man in his backstory as a Circus Brat when he shot the policeman who shot his elephant friend who had went into an amok run.
Sesame Street, Ernie tells Bert about his day at the zoo in an early skit. Ernie describes the day as largely uneventful, despite the chaos that resulted from several animals getting out of their cages. 
In the TV series based on the Tremors movies, the ass-blaster that was captured alive in the 3rd film escapes from captivity and menaces Perfection again.
Doctor Who: A pack of wolves escape the London Zoo in the "In The Forest Of The Night" episode due to the sudden, explosive growth of forests and run rampant around central London. A tiger also escaped and the wolves are far more afraid of it then any humans.
In the original MOTHER 1, this happened early in the game due to an alien invasion. It's your job to investigate and stop the aliens causing the trouble, all the while battling angry elephants and tigers.
Zoo Tycoon: Animals can be released by deleting a section of fence or they can escape on their own if you don't maintain damaged sections of fencing. Releasing the Tyrannosaurus in Dinosaur Digs can provide a little fun via easter egg. If the T-Rex destroys a one-person restroom you can see a person crying on the toilet (a reference to the lawyer's fate in the Jurassic Park films.)
Jurassic Park: The Game: it's set during and right after the first movie, and they have to deal with the escaped dinosaurs frequently.
On Corellia in Star Wars: The Old Republic the Coronet Zoo is damaged during the fighting and all the animals escape and start roaming around. As you arrive at the area, you overhear a man complaining about the "traumatizing" experience, but his daughter enjoyed getting so close to the animals and thought it was great. The game Codex says the rumors of a Rancor running loose are unconfirmed.
The cartoon "Jerry and The Lion" has a lion escaping from the zoo and Jerry desperately trying to keep him hidden from Tom. In the end he helps him get back on the boat to Africa.
In "The Little Runaway", a baby seal escapes from the circus and hides with Jerry, while Tom tries to catch him for the reward.
"Jerry and Jumbo" has a baby elephant falling off the circus train and ending up at Jerry and Tom's house. Jerry disguises him as a giant mouse to freak out Tom.
In "Down Beat Bear", it's a circus bear who dances whenever he hears music. Everytime Tom tries to report him, Jerry plays music and the bear makes Tom his unwilling dance partner.
In "The Missing Mouse", a white lab mouse escapes after ingesting an experimental explosive. Jerry paints himself white and messes with Tom, but Tom eventually finds out... just as the real lab mouse appears.
Mickey Mouse. In "Mickey And The Seal" a baby seal escapes from the zoo and unknowingly to Mickey and Pluto travels along to their house.
The cartoon "Donald Duck and the Gorilla" has a gorilla escaping from the local zoo and encountering Donald.
Superman Theatrical Cartoons. The cartoon "Terror On The Midway" features Superman attempting to stop the chaos created when several circus animals escape their cages and restraints, including a giant ape.
In the episode "Marge Gets A Job" a wolf escapes from Krusty's TV show. He then attacks Bart at school, who tries to warn Mrs. Krabappel, but just like in "The Boy Who Cried Wolf" nobody believes the previous liar anymore.
In "Two Dozen And One Greyhounds" Kent Brockman is reading the news:
In today’s news, a two-ton rhino escaped from the Springfield zoo. But zoo officials were quick to act, and Petunia, as she is known, is safely back in captivity. In other news, a three-ton rhino that escaped from the zoo last week is still at large.
Screaming Yellow Honkers: where Marge gets a Canyonero and develops a bad case of road rage. The climax is Homer accidentally releasing a pack of rhinos from the zoo and Marge having to use the SUV to corral them in and save her family.
This happens to Aang in Avatar: The Last Airbender in "The Tales of Ba Sing Sa" when he sets free all the animals in the Ba Sing Se zoo. To be frank, the zookeeper wanted to move them to wider open spaces.
Madagascar. The first Madagascar movie is about a gang of zoo animals who escape in the city because one of them - Marty the zebra - is bored of his zoo life, and the others follow him. They are then re-captured and released into the wild.
On the third of the Garfield Specials, Garfield on the Rough, an escaped panther stalks the woods where Jon, Garfield and Odie are camping.
Beverly Hills Teens. An entire circus is released when Chester accidentally gets some cotton candy on the cage controls.
Hey Arnold!. In "Cool Party," Curly goes crazy (again) after he's among those not invited to Rhonda's party, and acting on an impulse, frees all the animals from the zoo.
In 1874 the New York Herald printed a front page article saying that the animals from the Central Park Zoo had escaped and were rampaging across the city. It was a hoax story though.
George Orwellwrote a short story, ''Shooting An Elephant'' about a time when an elephant escaped from a circus and rampaged through a town in India. They called him out to kill it. By the time he found it, its rampage was over and it was eating quietly in a field. However, the whole town was gathered round, so he shot it. It didn't die, so he shot it again. Still didn't die. Third shot did it. Well, it took another half hour to die, but it collapsed.
Orwell: I often wondered whether any of the others grasped that I had done it solely to avoid looking a fool.
In 1994 a huge circus elephant called Tyke got furious after years of animal cruelty. He killed one of his trainers and then fled outside in the city, causing chaos and panic everywhere he went. He was eventually shot dead.
In 2007 a huge gorilla named Bokito escaped from a Dutch zoo. The media attention even inspired a short lived eponym "bokito proof", meaning a cage is strong enough to prevent the prisoner from escaping.
The BBC's frequent radio traffic reports throughout the day are usually sedate and rather predictable things, with the "usual suspects" and bottlenecks throughout the nation popping up with depressing regularity. But one day, the traffic reporter had to make an unrehearsed announcement and she was obviously reading blind from a report put directly into her hand. With clear and mounting incredulity, she reported that all traffic on a major road in Wiltshire had been blocked by a crashed lorry. from which two African rhinoceroses had escaped and were now charging up the dual carriageway playing chicken with the traffic. Police and the RSPCA were trying to contain the situation and specialist help was being sought in order to recapture the creatures. They had escaped from an animal transporter en route between two zoos, apparently. Happily they were recaptured alive, although no details were given. Damage ran into several tens of thousands of pounds.
The Zanesville, Ohio massacre of 2011 demonstrated that this is not usually a funny story when it happens in Real Life. A depressed man who also happened to have a huge collection of wild animals—bears, tigers, cougars, lions, and more—let all the animals out of their cages and then killed himself. The local police wound up shooting almost all of the animals.
There was a Real Life example at Omaha's Henry Doorly Zoo — one of the chimpanzees not only taught himself how to pick locks, but figured out how to hide the piece of wire he was using as a lockpick.
In the early 1970s, severe flooding in the Chicago area allowed two polar bears to swim out of their exhibit when their enclosure's dry moat filled with floodwater overnight. Luckily no one was harmed, but by the time their keepers arrived in the morning, the bears had eaten their way through every marshmallow, candy bar, and other tidbit at a nearby concession stand.
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