Runt

A Runt is the underdog, usually a degrading name given to someone smaller and weaker by Jerk Jock who picks on him/her regularly. In Real Life however, a runt is merely an animal who is usually last-born and its parents don't give a flying flock about it, having not invested much in it in the first place.

In fiction and literature, they are often the good guys, and almost always come out on top and gain genuine respect from the people who once ignored them and grudging respect from the ones who tortured them to no end.

When they're not good however, and they don't do something heroic to gain respect, these characters often grow up slogging about in their hatred and frustration, and eventually become a Big Bad. And when they Become Big Bad's, they are usually the cruelest, heartless, most fiendish of them all, out for blood and ruling with an iron fist. See also The Napoleon.

If they're not good or evil they're usually the Moe Moe or Team Pet or both, abandoned on the streets then found and owned by The Hero. If they're part of a gang or pack, they'll inevitably bring up the rear.

Film

 * In 101 Dalmatians, in both the animated film and the live-action remake, Lucky is characterized as the runt. Though he's the same size as his siblings, he was the only one born stillborn, and the one who either struggles the hardest with escaping from Cruella in the snow or suffering bad luck, such as a snow drift washing off his soot disguise in the climax.
 * How to Train Your Dragon (animation_ mentions in both the films and the show that Hiccup was a runt by Viking standards, a "walking fishbone" in his words. In fact, it's tradition to name any runt of the litter "Hiccup," sheep flocks and people included. Valka, his mother explains that it's because Hiccup was born premature, in the middle of winter no less.

Literature

 * In 101 Dalmatians (novel), Cadpig is the name given to the stillborn puppy that Roger revives. A sequel book reveals that she recovered well and grew up to have puppies as well.
 * A plot point in Charlotte's Web and all its adaptations; Fern saves a newborn runt piglet by begging her father not to kill it for being small. He explains that the piglet won't survive because his brothers and sisters keep pushing him away from his mother, but Fern cries and volunteers to rear "Wilbur". While Wilbur is a late-bloomer, thanks to Fern's bottle-feeding and care he becomes big enough, to the point that when his new owner, her uncle, decides to enter him into the county fair, he has a chance of winning due to his weight.
 * In Clifford the Big Red Dog's backstory, he was the runt of his litter. In fact, the animated series has the mother's owner warn Emily Elizabeth that the puppy will be a handful due to being so tiny he can fit into an adult's palm. Emily Elizabeth promises to train him and she loves him so much that he outgrows their apartment-- and the apartment building!
 * Fragile Things has "October in the Chair" cast a child like this in October's story; Donald is nicknamed "Runt" by his cruel older brothers, to the point that everyone has forgotten his real name, including his own parents. Donald decides to run away and have an adventure far away from his family.
 * In the How to Train Your Dragon series, it's later revealed that Vikings are supposed to either abandon their runts in the woods or set them out at sea, after naming them "Hiccup" if they belong to the Hooligan tribe. Hiccup I wasn't abandoned, but Hiccup II was and . If the runt comes back and survives, they were welcomed back into the tribe.
 * The fairy tale "The Ugly Ducking" has the title character treated this way by his family due to being tiny and "ugly" by duck standards. He becomes so upset that he runs away in the middle of winter, something that nearly kills him.

Live-Action TV

 * The Umbrella Academy adaptation has the other siblings constantly treat like this during their childhood, despite the fact that they're all the same time.  is the smallest, has no powers, and sticks out like a sore thumb.

Western Animation

 * BoJack Horseman reveals that Mr. Peanutbutter feels this way compared to his older brother Captain Peanutbutter, and he's perfectly fine with it, being the naive baby of the family. When Diane tells him on a vacation to the Labrador Peninsula that Captain Peanutbutter has been saying weird stuff about death to her, Mr. Peanutbutter at first gets angry on thinking his wife is projecting her Dysfunctional Family problems on his brother, but decides to go for a walk, and asks Captain Peanutbutter what is going on at a campfire.
 * Steven Universe
 * Jasper cruelly reveals to Amethyst that by Homeworld standards, she is an "overcooked runt" of a warrior Gem. Because she stayed underground for too long -- about 6000 years since the Famethyst emerged around that time-- she remains small compared to other Amethysts. It also means that she can't fight as well as Jasper can, and the latter proceeds to thrash the former.
 * was large by human standards, but by Homeworld standards?.
 * Tuca & Bertie reveals that by default, Tuca is considered this as the baby in her family. All of her siblings have respectable jobs and families, but she is a gig worker pursuing sobriety and a life's purpose. She and Terry, the oldest, have an argument about this, where Tuca asks for Terry to see how much she's grown and how how she used to be.

Real Life

 * Premature babies may be small owing to leaving the womb early. If they are severely underweight, they may be kept for observation in a neonatal ward.
 * If a piglet doesn't get enough milk, they will likely become the smallest of their litter.