In How to Train Your Dragon 2, what type of dragon is the giant alpha?

In How to Train Your Dragon 2, the giant alpha dragon is a Bewilderbeast.

The Bewilderbeast is an enormous dragon whose tusks, size, and ice-producing abilities make it one of the most imposing creatures in the series. The film presents the species as an alpha capable of controlling or influencing other dragons, placing it at the center of the conflict between Drago Bludvist and Hiccup’s allies.

A Bewilderbeast lives at Valka’s sanctuary before Drago captures and uses another one. The two giant dragons eventually confront each other, while Toothless challenges the older alpha and establishes himself as the new leader of the dragons. This contest is a major turning point in the second film.

The Bewilderbeast is not the same creature as the Red Death from the first film. The Red Death is the giant dragon defeated at the climax of the original movie. Toothless is a Night Fury, a different species that becomes an alpha through the events of the sequel.