"Shouldn'ta lost me temper ...but it didn't work anyway. Meant ter turn him into a pig, but I suppose he was so much like a pig anyway there wasn't much left ter do."
-- Hagrid, after giving Dudley a pig's tail (PS4)
References from the canon
- Harry often thought of Dudley as a "pig-in-a-wig" because he was a glutton and a bully who always got his way with Aunt Petunia (PS2). But when Uncle Vernon insulted Harry's parents and Dumbledore, Hagrid retaliated on Dudley with the pig's-tail jinx. Hagrid later told Harry the spell was a mistake - he actually meant to turn Dudley completely into a pig (PS4).
- The Dursleys had the pig's tail removed at a hospital so Dudley could be accepted into Smeltings school (PS6). They explained it to the doctors as an out-of-control wart (Sch2)
The spell is unnamed in canon and Hagrid apparently casts it non-verbally.
- In the movie version, Hagrid gives Dudley the pig's tail because he is gobbling up Harry's pink birthday cake. In the book, Dudley never touches the cake, and the jinx is performed as retaliation for Vernon calling Dumbledore an "old crackpot."
- Hagrid was not being a good role-model for Harry in attacking a Muggle child as a way to punish Vernon and Petunia. Indeed, as soon as they reached Diagon Alley, Harry wanted to buy a book about dark magic so he could curse Dudley himself. Hagrid discouraged him, but only because Harry is underage: "I'm not sayin' that's not a good idea, but yer not ter use magic in the Muggle world except in very special circumstances" (PS5)
- J.K. Rowling has taken some criticism for making Dudley a target of fat-shaming, not just from Harry, who has good reasons to resent Dudders and is a child himself, but from Hagrid, a huge half-giant man with pockets full of sausages and cake, who has to eat almost constantly. At that point in the series, Hagrid knew nothing about Dudley except that he was Vernon's son.
- In Goblet of Fire, the Weasley Twins decided to get Dudley back for bullying Harry, also by appealing to his gluttony for candy with Ton-Tongue Toffee that deformed his tongue, and caused their father Arthur to chastise them (GF4, GF5). The bullying from Dudley and from Aunt Marge (another "fat" character who was "blown-up" by Harry) eventually became known to the teachers and Dumbledore through Snape's Occlumency lessons in 5th year when he saw Harry's memories (OP24).
- The author said the Dursleys told doctors the pig's tail was a "wart that got out of control." That seems to be a pun on "Hogwarts," with Dudley being the hog (Sch2).
The author's interpretation of the pig's tail jinx scene are interesting and in some cases contradictory:
'I like torturing them (the Dursleys)....You should keep an eye on Dudley. It’s probably too late for Aunt Petunia and Uncle Vernon. I feel sorry for Dudley. I might joke about him, but I feel truly sorry for him because I see him as just as abused as Harry. Though, in possibly a less obvious way. What they are doing to him is inept, really. I think children recognize that. Poor Dudley. He’s not being prepared for the world at all, in any reasonable or compassionate way, so I feel sorry for him. But there’s something funny about him, also. The pig’s tail was irresistible 'Cinescape Interview, 16 November 2000.
Some time after that interview, JKR wrote an essay about how society expects girls to be model-thin and judges women based on weight, something that makes her angry:
"Fat" is usually the first insult a girl throws at another girl when she wants to hurt her,' I said; I could remember it happening when I was at school, and witnessing it among the teenagers I used to teach. ....how strange and sick the 'fat' insult is. I mean, is 'fat' really the worst thing a human being can be? Is 'fat' worse than 'vindictive', 'jealous', 'shallow', 'vain', 'boring' or 'cruel'? Not to me...(JKR)
That statement caused even more pushback against her in fandom, since there's no doubt that Dudley and several of the Slytherins (Marcus Flint, Vincent Crabbe, Millicent Bulstrode, and Professor Horace Slughorn) are judged and ugly or evil by Harry on first sight simply because they are overweight compared to himself with his "skinny legs." Also some readers with weight problems did not appreciate the comparison between being "cruel" or "vindictive" and being of a certain size, since cruelty is a choice but weight might be caused by genetics or an illness. Again, the author lashed back at critics on her Official Site:
I thought of listing all the many characters in the Harry Potter books who are on the plumper side, to demonstrate what a very diverse group of personalities they are, how they include several of my most important, admirable and lovable characters, and how 'overweight' in no way equates to 'bad' in my fictional world... (JKR)
Again, that answer did not settle the question, since we know JKR admires Hagrid (good) but dislikes Dudley (bad, but pitable). So that leaves us back where we started. There still seems to be no good reason for having one beloved "fat" character shaming another, especially a child, and turning him into a pig. While it may seem like justice for what Harry endured, it's also childish and uncalled-for.
While trying to get under Dudley's skin in OP1, Harry tells Dudley that he resembles a pig walking on two legs. Perhaps Harry knew a vague allusion to the humiliating pig tail incident would would be an effective way to irk his cousin.