Per my friend who works at DreamWorks Animation:

”ATTN: Anyone who gives a shit about How to Train Your Dragon 2! There will be a new trailer coming out soon. DO NOT WATCH IT. 

The bean counters at Fox decided to spoil the main mystery of the movie in the first minute of the trailer. EVERYONE in the production, including the director and producers, fought them tooth and nail on it. We lost. 

Trust me when I say you will enjoy the movie. But you will enjoy it a lot more if you avoid the trailer.”