Troublemaking New Pet

You've just noticed that your pet is seemingly lonely, so what's the best thing to do? Get a new pet that might be a good friend for the current pet. However, just as soon as you leave the room, the net pet actually turns out to be very troublesome and spoiled, tearing up the rugs and the curtains, spilling milk all over the floor, and knocking over the trash.

When you come back in the room, you become surprised at the mess and become quick to blame the old pet for causing the damage. The pet tries to insist that he did not do it, but you don't listen. The old pet takes matters in his own paws to expose the new pet for the fraud that he is. Once the new pet has been exposed for what he really is, he is immediately disposed of and the old pet is forgiven.

Also, there are some variants of this trope where instead of a pet, it's when a person tries to make friends with another character who also does a lot of misdeeds to get the other in trouble.

Compare: Bitch in Sheep's Clothing and Clear My Name.

Examples of Troublemaking New Pet include:

Advertising

A Polaroid commercial had a dog who was blamed for getting into the trash, despite the fact that he was the one who didn't do it. The dog then discovers that it was actually the cat who got into the trash and pinned the blame on him, so he uses the titular camera to take pictures of the cat to prove his innocence to his owner.
This Pepsi commercial had a dog eat his owner's sandwich and drink his Pepsi before making it look like that the cat did it.

Anime and Manga

A Johto episode of Pokémon had Ash and friends come across a seemingly cute and innocent Teddiursa, whom they temporarily take under their wing. However, when Ash, Misty, and Brock are not around, Teddiursa is not as innocent as he seems as he frames most of the Pokemon (Totodile, Chikorita, Psyduck, and Bulbasaur) for eating the food, which it actually did itself.

Film

A scene in Cats and Dogs had Mrs. Brody adopt a seemingly cute kitty which Lou the beagle is uncomfortable with, until it is revealed that the kitty was actually an agent for Mr. Tinkles. He immediately frames Lou by using a metal ball containing fake dog poop and dumps it on the floor, leading Mrs. Brody to think that Lou defecated on the floor, when really he did not.
In Lady and the Tramp, Aunt Sarah stays to take care of the baby while Lady's owners are away. She brings along her Siamese cats, who quickly get up to mischief. When Sarah comes in and sees the mess—along with her "innocent little angels" acting hurt—she blames Lady and puts a muzzle on her. The Siamese cats on the other hand, were both never seen, heard, or mentioned again after this.
- Aunt Sarah later does this to Lady's boyfriend Tramp, whom she thinks was attacking a baby, and immediately has him taken away by the dogcatcher (it was the rat that was attacking the baby, Tramp was actually protecting him).
In The Ugly Dachshund, the dachshunds act like this to the Great Dane, despite him being the "new" pet. After spilling paint, they're even shown cleaning themselves up so their masters won't notice they did it.

Western Animation

An episode of SpongeBob SquarePants had SpongeBob buy another pet as a friend for Gary, but the pet causes a lot of trouble by tormenting Gary, but since Spongebob is an expert of Comically Missing the Point, he thinks that Gary is roughhousing with the pet, even in the end when he blames Gary for all the damage.
Used without a new pet in the Foster's Home for Imaginary Friends episode "Everyone Knows It's Bendy". The titular friend is apparently wrongly blamed for all the misdeeds that he committed at his old creator's home, but once he's been welcomed to Foster's, he actually turns out to be a complete Jerkass who promptly gets Wilt, Eduardo, Coco, and Bloo into various kinds of troubles that he did himself, while still mooching to Mr. Herriman and Frankie under his innocent facade.
The The Real Ghostbusters episode "Victor, The Happy Ghost" had the Ghostbusters adopt the titular ghost (an obviously blatant Expy of either Casper the Friendly Ghost or a Dr. Seuss character), much to Slimer's discomfort. However, Victor is not as innocent as he seems as he causes a lot of trouble for the Ghostbusters; touching one of Egon's inventions, eating the whole box of jelly donuts, closing the hood of Ecto-1 on top of Ray while he was working on the car, etc., leaving Slimer to be blamed for all the misdeeds that Victor did himself.
In the Tex Avery cartoon "The Cat Who Hated People", the cat is taken home to make friends with the family dog. The dog starts beating him up, but when the lady of the house arrives, he quickly gives the cat an axe, douses himself with ketchup and plays dead, and the poor cat gets the rap.
In the Tom and Jerry short "Tom's Photo Finish", Tom is almost caught raiding the fridge and sets Spike up to take the fall, which gets the poor dog taken outside. Unfortunately for Tom, Jerry has incriminating evidence, and Hilarity Ensues as Tom tries to cover it up.
- Another cartoon, "Triplet Trouble", has Mammy Two-Shoes take in three kittens who then make mischief the moment she leaves the room. Tom and Jerry have to join forces to get even with them.
In the Looney Tunes short "Hare Force", Bugs Bunny is taken in by an old lady and locks horns with her jealous dog.
The House of Mouse short "Pluto's Kittens", which is about Pluto being kicked out of the house by Mickey Mouse after his conscience persuades him that the poor dog was the troublemaker and not the titular cat trio.
In her debut episode in Rugrats, Fluffy is like this to Spike.
In an episode of Eek! The Cat, the title character's owners Wendy Elizabeth and JB adopt a pink bunny who turns out to be a giant gray killer in disguise and causes a lot of damages in the house. Eek's owners think it was him doing it and punish him several times.