There are so many differences between the movie and the book for How to Train Your Dragon . . . so here goes . . .
While both the book and movie have Vikings and dragons, the relationship between the two species is very different. In the book, the dragons are more like working pets, going out and hunting for their Viking owners. Only dragons are not the most cooperative creatures, due to their high intelligence apparently, and hard to control.
Hence, the initiation ritual in the book. The ten boys are sent into a cave where there are several thousand baby dragons and must retrieve one and, after bringing the dragon home, train it in time for the big festival.
In the movie, the Viking/Dragon relationship is antagonistic and when the youth are initiated it’s in an arena where they first train to learn about how to fend off various dragon attacks. And then there’s the girl, Astrid, who is also trying to be a Viking warrior. I suppose this was Hollywood’s way of making the movie more universally appealing and I can’t argue that it didn’t work. However, let’s be honest: it’s not very historically accurate.
Nevertheless, when Hiccup happens to seriously injure one of the most fearsome types of dragons, he begins learning how to train a dragon. Albeit, his fearsome dragon can’t fly (it’s wounded, after all) and at first he thinks it’s toothless, which is why he calls it Toothless.
And that is what book Hiccup calls his dragon because his dragon actually is toothless. And very tiny. And not of the most fearsome breed. Throughout the book, Hiccup tries to manipulate the dragon through various means to win it over but the dragon is not easily fooled. (Remember, they’re clever creatures.) This is different from movie Hiccup who actually is inspired by compassion to find a way to connect with the dragon.
This is why I feel the movie is better than the book. I prefer the compassionate Hiccup to the frustrated and manipulative one. Both are fearful and both prove to be courageous. Even Toothless is far more likable in the movie than he is in the book where the tiny Toothless is petty and narcissistic.
Both have a very large dragon that must be defeated but how this is accomplished is completely different. In the movie, the humans and smaller dragons work together to bring the dictatorial dragon down and, at the end of the movie, they are living in harmony more cooperatively than in the book where the dragons are clearly servile (if not submissive) to their human masters from beginning to end.
I suppose the end result is the same—humans and dragons live together in relative harmony. Still, it seems to me that the movie suggests a deeper relationship between the two, that the humans and dragons have a mutual respect and work together whereas one gets the feeling in the book that the next time a really big dragon washes ashore, the smaller dragons are going to run, or fly, to the hills.