An ancient evil that has been awakened. It could be a powerful sorcerer from the past who wants to control all the dragons for his own dark purposes. He might use magic to manipulate the dragons and turn them against Hiccup and his friends.
A rogue dragon rider who has different ideas about how to treat dragons. He might see them as tools for power rather than friends like Hiccup does. This rider could be causing trouble for Hiccup and his tribe by stealing dragons or using them to attack other tribes.
A rival tribe that is jealous of Hiccup's bond with the dragons. They believe that the dragons should be under their control and they start a conflict with Hiccup's tribe. Their leader could be the main antagonist, constantly devising plans to undermine Hiccup and take the dragons for themselves.