Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: How to Train Your Dragon
The falling scene, especially once they pass the peak of the mountain.
The Green Death's introduction. You HAD to feel seriously sorry for that poor unfortunate Gronckle.
The Green Death, a dragon the size of a very large hill, taking flight and ascending into the sky after Toothless and Hiccup. Having it emerge from the earth is terrifying enough, but seeing something that looks like the unholy spawn of Fenrir and Jörmungandr take to the air and disappear into the clouds left one awestruck. That's the point where the Vikings probably thought that Ragnarok has started and the world was about to break.
Not to mention seeing it breathe fire when it can't keep track of where Toothless is flying. When most of the other dragons have been shooting quick bursts of fire, watching the Green Death freak out and unleash this continuous stream of flames through the sky is just terrifying.
Also some of the details on the various dragons in the handbook.
Hiccup: Eats its victims... burns its victims... buries its victims, chokes its victims, turns its victims inside-out... Kill on sight.