The following are some of the most notable differences between the original How to Train Your Dragon book and the movie adaptation.


Books Movie
In the book, Hiccup's best girl friend is a fierce sword fighter named Camicazi. In the movie, Astrid is Hiccup's love interest plus an original character to the film. Camicazi never appears unlike the book.
In the book, Hiccup's dragon, Toothless, is a tiny, toothless, emerald green Garden Dragon. Later it is found out that he is a Seadragonus Giganticus Maximus In the movie, Toothless is a jet black Night Fury, the rarest of all dragons. He has retractable teeth and his size is large enough to serve as a flying mount for both Hiccup and Astrid.
In the book, Fishlegs is smaller and scrawnier than Hiccup but he never gets to be accepted by the other Viking teens. He is not very wise about dragons but he is Hiccup's best friend and sidekick. In the movie, Fishlegs is bigger and chubbier than Hiccup. Unlike in the book, he is accepted by the other Viking teens, even though they were annoyed by his spoutings of information on dragons, and he and Hiccup aren't friends, but he doesn't pick on Hiccup like the other teens did.
In the book, Snotface Snotlout's main sidekick is Dogsbreath the Duhbrain, a giant ape-like boy who pushes Hiccup around. In the movie, Snotface Snotlout has no sidekicks, the closest comparison being the twins Ruffnut and Tuffnut. Dogsbreath the Duhbrain is never mentioned and he is never to appear in the movie.

In the books, all of the Vikings are trying to train a dragon in order to become accepted as part of the tribe. However it is revealed that in the past the vikings were at war with the dragons similarly to the film. Later the vikings go back to war with the dragons in the later books.

In the franchise, the Viking tribes were at war with the dragons who were stealing their livestock. By the end of the film, the dragons are now invited to live in the village.

In the books Alvin the Treacherous is Hiccup's relative as they share a common ancestor in Grimbeard the Ghastly. Alvin is also skinny. though more muscular in later books an looses body parts as the series progresses. In the franchise, Alvin is a lot bulkier and isn't related to Hiccup by blood. He was however a childhood friend of Hiccup's father, Stoick the Vast before his banishment.
In the books, Snotlout is Hiccup's cousin and rival for the title of chief of the Hairy Hooligans. Snotlout also hates Hiccup and wishes to kill him to be next in line for chief. This sub-conflict is later resolved when Snotlout sacrifices his life. In the franchise, Snotlout isn't related to Hiccup by blood and comes from a different family. Snotlout also doesn't really hate Hiccup, he just looks down on him and mocks him. At the end of the first film Snotlout and Hiccup become friends.