Occasionally, there are alleged sightings of animals waaay outside their natural environment
. A kangaroo in North America, for example, or a big cat in Australia.
These animals, until their existence is confirmed, are often classified as Cryptids
. Sometimes, a work may have an episode focussing one one of these, determining a) Whether the animal is real, or if there's an alternative explanation, b) If real, how it came to be in that environment, and c) What can be done about it.
A popular explanation is that they escaped from a zoo or travelling circus sometime in the last century, and set up a breeding population. Alternatively, the entire thing may be a hoax, often set up to cover a murder or other crime.
An alternative approach has the animals being introduced to their new home at the start of the story, with the plot focusing on the (often destructive) effects of the animal in its new ecosystem. This variant is somewhat rare in fiction, although it has happened multiple times in Real Life
A third variation has the animals' presence not be the main focus of the plot, but once the characters realize that they're not supposed to be there, it serves as a vital clue in a broader mystery.
- The very first story of the Silver Age Hawkman has him realize that a bird depicted in a newly created museum diorama is actually from his home planet, and talks to the person who made the diorama to discover where the alien shapeshifter Byth is hiding.
- The deceptively cheerful-looking picture book, Cat on the Island, by Gary Crew, is based on the true story of the introduction of domestic cats to a small island in the Pacific, where they quickly wiped out an entire species of ground-dwelling finches.
- In the Ciaphas Cain novel Caves Of Ice while searching some mines they're guarding (having found out several workers had disappeared recently and seeing an ideal excuse to keep his head down in an environment he loves; tunnels) they find a nest of desert dwelling Ambulls have been feeding on the miners, which (as you would guess from the title) is a little out of place. It turns out that they came through a necron warp portal.
- Used in an Encyclopedia Brown mystery, where penguins are shown in a display of a North Pole explorer.
- In the novel Sewer, Gas & Electric, the submarine piloted by the eco-pirates was originally commissioned by Howard Hughes, who'd intended to make use of it in a drug-addled scheme to release kangaroos in the central United States. He'd wanted their presence to be a mystery that would tie up federal resources for years, trying to solve it, but in actuality all the displaced marsupials did was to devour a local pot-farmer's entire crop. This is one of the less bizarre events in the novel.
- The Diatrymas in the New Forest (first mentioned in Lost in a Good Book) are subsequently mentioned in the sequels. The infamous Goliath Corporation cloned them, officially denied doing so and later officially apologized (wholesale, along with many other things); they go from unsubstantiated rumour to sightings near Salibury, with Stiggins the Neaderthal tracking them down from his cover job at Acme Carpets.
- The Wolves of Willoughby Chase
The action of this book takes place in a period of English history that never happened - shortly after the accession to the throne of Good King James III in 1832. At this time, the Channel Tunnel from Dover to Calais having been recently completed, a great many wolves, driven by severe winters, had migrated through the tunnel from Europe and Russia to The British Isles.
- An episode of The Simpsons, "Bart vs. Australia", references the Real Life case of the introduction of the cane toad in Australia.
There are 200 million in North America descended from 60 to 100 birds released in Central Park, New York by Eugene Schieffelin. He was president of the infamous American Acclimatization Society which tried to introduce every bird species mentioned in the works of William Shakespeare to North America in 1890, and this turned into a terrible environmental disaster...Starlings are among the worst nuisance species in North America. The birds travel in enormous flocks; pose danger to air travel; disrupt farms; displace native birds; and roost on city blocks. Corrosive droppings on structures cause hundreds of millions of dollars of yearly damage. In 2008 the U.S. government poisoned, shot or trapped 1.7 million, the most of any nuisance species.