When Hiccup is reading about the dragon types from the manual at some point some of the dragons on the pages are shown as animated (very little general body movements on the manual pages) after he startles by the lightning. It looks like his imagination and fear gets to him. But why did they show those animated dragons just for a brief duration and then finished doing it? Was it referencing some older dragon related animation movie or was it just a little gimmick to make the younger viewers get into the mood?
Basically the purpose of that was to gimmick to gain viewer's attention. Just to show what those dragons can do.
If you can check it out most of the dragons in the book were not shown in the movie.
Like below are two examples from the dragon book.
These were not in the movie, so they animated them to show the details about what the past dragons were.
Hope this answers your question