Some rich people give their fortune to their relatives when they die (though in the media, it's often On One Condition). Some leave their fortune to charity. And some eccentric types leave their money to... their beloved dog. Or cat, or fancy rat, or parrot, or whatever.
That's the concept behind the Pet Heir plot. Often Hilarity Ensues as the new caretakers of the pet (usually the protagonists) struggle to keep the pet alive despite the efforts of unscrupulous sorts (frequently family who were deemed Inadequate Inheritors) who are next in line to inherit should "anything unfortunate" happen to the pet in question. Why the alternative heirs are in such a rush to inherit, and can't simply wait for the pet's natural lifespan to expire, is seldom addressed.
Legally, of course, animals cannot own money or property, and domesticated animals are, themselves, property; but a testamentary trust for an animal's care could be set up, and all the estate's assets could go to that trust. Which would leave the trustee in control of the assets.
There are cases of Truth in Television for this trope, oddly enough. Or not, if the pet in question is a parrot or a similarly long-lived pet critter.
Comparable to Caligula's Horse, where the person not only gives their inheritance to their pets, they even give them senior positions in jobs.
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The board game 13 Dead End Drive is about heirs in line to inherit the late Aunt Agatha's fortune trying to bump each other off in a booby trap-filled mansion, and escape while they're the current heir. One of the playable characters is, you guessed it, the old lady's cat. In the revised version, getting the cat out of the house will give you a game-winning advantage.
The Lucky Luke comic The Inheritance of Rantanplan has the guard dog Rantanplan inherit a fortune from a former convict, and becomes a target for the Dalton Brothers since Joe Dalton is next in line.
Pooch Cafe: Poncho's brother Orlando is this, having inherited vast fortune from his deceased owner, an oil baron.
There is one Soviet Russian propaganda movie where a rich American lady dies in a car crash, and her dog inherits her entire fortune. Then the dog goes on to become a socialite, a congressman and then even the President. All because of the money.
Films — Animation
In The Aristocats, the title felines are to be the main beneficiaries of their owner's will in order to keep them looked after. Edgar, the butler, eavesdrops on the conversation and then decides to try to off the cats in question as he was to receive the rest when the cats died. Incidentally, it did "address" the issue of him just waiting for the cats to die: he assumes that each cat really has nine lives. And, for some reason, that the four cats will live consecutively. He's not very bright.
Ironically, she implies at the end that she intended to leave the butler a sizable sum himself.
In The Fearless Four, Aunt Wanda combines this with On One Condition: her estate goes to her family, but only if they take care of her cat Gwendolyn, who inherited Aunt Wanda's "treasure," a pair of incredibly valuable jewels. The family decides to kill Gwendolyn when they get tired of caring for her and frustrated that she won't tell them where the jewels are.
In Millionaire Dogs, a woman left her fortune to her several animals while her nephew and her niece got nothing. The evil duo then tricked the animals out of the house and tried to keep them out for 48 hours in a row so they could invoke an obscure law to claim the inheritance to themselves.
In the film Gordy, an old man disinherits his daughter in favor of a talking pig because of her decision to become a model. This happens just after the pig and his human friend have wrecked her career by innocently causing her trap for them to backfire.
Quite a few live-action Disney films, especially from between the original Disney and Michael Eisner, do this.
The 1951 film You Never Can Tell started with a dog inheriting a fortune and immediately being killed and his caretaker suspected. The dog's spirit is sent back to earth as a "humanimal", detective Rex Shepherd (Dick Powell), to, errr sniff out the crime.
In Soul Music, one of Susan's first "customers" as acting Death is a grumpy old man who leaves his fortune to his cat instead of his ungrateful, parasitic relatives. Of course, he hates the cat too, so he doesn't set it up any kind of protection from said relatives.
This forms a major part of the plot in Making Money. Near the beginning of the book, Topsy Lavish leaves her shares in the Ankh-Morpork Royal Bank to her dog, Mr. Fusspot... and leaves the dog to Moist von Lipwig, with a note that Moist will be targeted by the Assassins' Guild if Moist should decline to take care of the dog.
There's hints in the book that this wasn't done out of eccentricity, but because she wanted Moist to run the bank. She couldn't give him the shares directly, since he couldn't be both Postmaster General and Master of the Mint (which was part of the Bank).
The adult novel The Mystery of the Fat Cat by Frank Bonham featured a variant of this trope. A wealthy old lady left her estate and house to her cat for the rest of its natural life, under the care of a trustee; after the cat's death the house is to go to the city for a park. The book opens with the protagonists becoming suspicious of how long the cat has lived under the guardian's care, and they start to investigate.
In H. Allen Smith's 1946 novel Rhubarb (filmed in 1951), a cranky millionaire leaves everything—including a major league baseball team—to his cat Rhubarb. The team's players and the millionaire's disinherited daughter are among those who have problems with this.
One of the endings for the interactive book "The Dandee Diamond Mystery" has the benefactor's parrot inheriting the diamond.
To Catch a Leaf, by Kate Collins, has a tabby that inherites its owner's mansion and all the contents thereof, including millions of dollars worth of art and furniture.
In Francis M. Nevins' "Dogsbody" a rash of pet poisonings was attributable to a disgruntled nephew who felt he was more deserving of his uncle's fortune than the formerly-stray dog who was liked better than the deceased's relatives.
In an episode of Black Books, Bernard's landlady died and left the building to her cat. Which led Bernard to try and hire someone to off the cat.
On Rescue Me, Tommy's father marries a rich woman and hopes to inherit her fortune when she suddenly dies. After an extensive spending spree, the family finds out that he inherited $50,000, and the rest was left to her various cats.
David Cross had a hilarious bit once about rich people who die and leave their vast fortunes to their cats.
"And all my money goes to Miss Cinderella, so she can live in luxury, like the princess she is. Also, fuck the homeless."
Housepets! - The wealthy Henry Milton leaves his fortune to his six pet ferrets.
This is part of the concept of Catscratch: the main characters are a trio of talking cats who inherit their owner's vast fortune after her death.
The Droopy cartoon Millionaire Droopy revolved around this, with a bulldog rival of Droopy's trying to bump him off or otherwise get rid of him.
In a flashback episode of The Simpsons, Marge and Homer are looking for a new house, and one of the houses they look at is full of cats, who apparently inherited the place from its late owner. In fact, the Realtor even goes so far as to say that not only do the cats own the house, Marge and Homer would be their tenants.
Another episode of The Simpsons has Mr. Burns naming his pet tortoise as a benefiary of his will.
Garfield once inherited the Klopman Diamond but the bad luck curse surround it led Garfield into selling it and the money the buyer gave him for it went into the repairs Garfield's home needed because of the curse.
One Mighty Mouse cartoon featured a wealthy couple who left their fortune to their mice. The couple's nephew tried to have their will invalidated but the judge ruled that they were in perfect mental condition (albeit one can doubt that while taking a look at the will). The nephew didn't take it well and ended up being arrested. He soon escaped and decided to steal his uncle and aunt's mansion by dragging it with his bicycle. Fortunately Mighty Mouse restored it to its rightful place.
Precious Pupp once believed he'd inherit a fortune and a dog named Mauler would get everything if something happened to Precious. After Mauler tried (and failed) to kill Precious several times, Precious' mistress got her new glasses and found out she misread the will. Mauler was the main inheritor and Precious was the secondary one. Precious' Muttley-like laughter was a sign of upcoming payback.
Sylvester is left a fortune in the Looney Tunes short "Heir-Conditioned".
Some species of pets in particular will almost require this. If an adult human buys a young parrot, that bird will likely outlive its owner. Not crazy, just responsible with a animal that can live 60-70 years.
Tortoises can live over 100 years, with documented cases of 150+ years. Responsible owners will make provisions for their tortoises' care in case they predecease their pets.