In many works of fiction, there comes a time when a character will be walking along when, suddenly, they see a cardboard box on the side of the road. Inside said box is inevitably an abandoned Cute Kitten or Precious Puppy, left by an irresponsible pet owner, just begging to be taken home. For some reason, they never seem to leave the box even if the cats, especially, could jump out easily.
A kind passerby will fawn over or otherwise react with sympathy for the abandoned animals and may even adopt them; more callous characters will ignore them or mistreat them outright. The animals themselves may or may not get adopted; luckier ones find their forever homes on the sidewalk while less fortunate puppies and kittens may often have to go through further hardships before finding their happy endings. In Real Life, if you have a pet that you can't take care of, or your situation has changed and you can no longer take care of your pet, don't just leave it in a box; surrender it to a shelter. They'll help the pet find a good, loving home, and care for it in the meantime. (Also, unless you're a licensed breeder, spay and neuter your pets, so you won't have an entire litter of puppies or kittens you need to decide what to do with.)
A popular method of introducing a Loyal Animal Companion, or A Pet Into The Wild if the poor animal doesn't get adopted right away. Sister trope to From Stray to Pet and Doorstop Baby. See also Girl in a Box, which uses boxes to introduce a likely major character.
- In one episode of Doraemon, Doraemon helps Nobita secretly care for a box of puppies/kittens. After some time travel and thought process acceleration, the two returned to find that the dogs and cats became humanoid, gained the ability to talk, and built a sophisticated civilization with high technology. Apparently, the now-humanoid dogs and cats have decided to leave the Earth to another planet because they've predicted that the Earth will be facing a world-wide disaster.
- In one chapter of Kimi ni Todoke, Sawako and Kazehaya find an abandoned puppy by a river.
- Massugu ni Ikou: In episode 4, Mametarou and Hanako come across a puppy in a box. They end up acting as Parental Substitutes until it is taken home by a girl.
- Sakamoto from Nichijou. Nano has no choice but to leave him behind when she first finds the cat in a box, only to find him again at home courtesy of the professor.
- Kicks off the plot of Nukoduke. In the first chapter, Yuuya found Kei and Sasame abandoned in a box on the street, and decided to adopt them.
- In the "Go West Young Meowth" episode, it's shown that Meowth was abandoned as a kitten in a nest. He grew up on the streets with no parents, gang, or trainer.
- Plus, a Bulbasaur was abandoned by its trainer, and left in a box in the river.
- At the end of the second story arc of Rascal Does Not Dream of Bunny Girl Senpai, Sakuta & Mai find a kitten abandoned in a box while walking home. Sakuta ends up taking it back to his apartment, as he and his sister Kaede already own a cat.
- My Roommate is a Cat: Haru and her four brothers were originally left in a cardboard box, and had to fend for themselves out on the streets.
- In Mahou Shoujo Site, Aya befriends a stray kitten who was abandoned in a box. She gives it a collar and food, but she's unable to take it home. Her bullies find out about the cat and kill it just to spite her.
- This is the backstory for a Dex-Starr/Dexter, a cat that joins the Red Lanterns in Green Lantern. Dexter & his siblings are shown in a box labelled "FREE" on the day of Dexter's adoption. When his adopted owner is killed in a home invasion, Dexter is thrown into the street and nearly drowned by two thugs who throw him into the river - at which point he is recruited into the Red Lanterns and becomes Dex-Starr.
- My Little Dashie: A man finds a filly Rainbow Dash and takes her home.
- This trope is what kicks off the plot in the Homestuck fanfiction Unwanted Free Ugly Troll: in an AU where trolls are kept as pets, Dave finds Karkat abandoned in a cardboard box and ends up adopting him.
- In My Huntsman Academia, Izuku finds a puppy stuck in a box alone while hanging out in Vale. He takes it to the pet store and the vet, upon which he learns that it's a girl. He then takes her home to his dorm, upon which Team MNVW instantly falls in love with her. He ends up surrendering the puppy, now christened "Lulu-Bell, Lord of Thunder", to the shelter because he isn't sure if they'll have time to take care of her, but later returns to formally adopt her.
- In In Sickness Harry finds a kitten in a soggy cardboard box in an alley.
- In Balto II: Wolf Quest, Balto and Jenna's puppies are given away in a box. One of them (Aleu) however doesn't get adopted because her looks betray her wolf-dog heritage.
- Inverted in Mr. Peabody & Sherman; the canine Peabody found the human Sherman as a baby inside a cardboard box in an alley.
- Oliver & Company opens with a box full of kittens being sold on a Manhattan sidewalk. All the kittens are sold except for one - our hero, Oliver - who is callously left in the box on the rain.
- In Rio, Blu the macaw was discovered by Linda in a box on the side of the road.
- In The Secret Life of Pets, this is how the protagonist is found by his owner.
- This is Riley's backstory in Homeward Bound II: Lost in San Francisco. The boy he was bought as a present for didn't want him, so the parents left Riley in the gutter. Unlike many examples, Riley is not adopted by a kind passerby; he becomes a street dog with a deep distrust of humans.
- In the first book of the All American Pups series, New Pup on the Block, the others meet Rosie when her box is tossed out of a moving car.
- Sphynx the cat from A Certain Magical Index was left in a box◊ and adopted by Index.
- Played for Laughs in My Teen Romantic Comedy SNAFU. A little kitty is left in a box to get Saki Kawasaki's attention, but failed. Instead, Yukino Yukinoshita drops her coldness to stay in front of the box meowing with the kitty.
- In the video for BIGMAMA's "Crystal Clear," the main character finds her cat this way; she and the cat eventually grow so close that she researches new methods of space travel to break into another plane of existence to meet with the cat in the afterlife.
- The children mobile game Little Kitten - My Favourite Cat is about a kitten abandoned in a box that gets into a house and is adopted, the game is about to pet and play with the kitty (called as Crumbs) in an interactive house.
- In We Bare Bears, when the bears are still cubs, they try to find a family by sitting in a cardboard box with "Free Bears" written on it.