In How to Train Your Dragon, the giant dragon defeated at the end of the film is the Red Death.
The Red Death is a massive dragon that controls the other dragons attacking Berk. It lives in a volcanic nest and forces smaller dragons to bring it food. This explains why the dragons raid the Vikings’ supplies even though they are not acting independently in the way the villagers assume.
Hiccup, Toothless, and the other young Vikings confront the Red Death near the film’s climax. Toothless helps Hiccup maneuver around the giant creature, while the young riders work together to survive the battle. The Red Death is ultimately defeated when Toothless uses a powerful burst of fire after Hiccup creates an opening.
The Red Death is separate from the Bewilderbeast in How to Train Your Dragon 2. Both are enormous dragons connected with control over other dragons, but they belong to different films and have different designs and roles.