Troublemaker

In How to Train Your Dragon 2, the Great Troublemaker is a gigantic dragon, effectively the "king of dragons." The film features two such dragons: one owned by Valka (white, more noble), and the other by Drago Bludvist (gray, aggressive; also known as the Evil Troublemaker).

Appearance
Immense size. The Mischief-Maker is one of the largest dragons in the universe: it has been compared to an entire island, and is classified as a 10th-tier Leviathan.
Tusks. Long, powerful tusks grow from both sides of its head—with them, the Troublemaker can ram enemies and even pierce the armor of other dragons.
Spikes. The front of the body is covered with blunt spines; on the neck and head, they form a semblance of a mane, and small spines on the muzzle resemble eyebrows and a beard.
Wings and tail. The wings are relatively small for such a giant, so the Troublemaker can barely fly, but uses them as fins in the water. The tail is flexible and helps it maneuver underwater.
Muzzle. Flat, with a rounded tongue and small teeth; the mouth resembles a fish's, the lips are reminiscent of a mammal's—thanks to this, the Troublemaker is able to "blow" jets of icy water in a unique manner.
Coloring. Troublemaker Valka is pure white (his design is inspired by a polar bear), while Troublemaker Drago is dark gray (giving him a more somber and oppressive look).
Capabilities
Ice Breath. Instead of fire, the Mischiefmaker unleashes powerful streams of icy water that explode on impact, covering the target in sharp, greenish ice spikes.
Dragon Control. Using ultrasonic waves and a special vibration from the spikes on his head, the Mischief-Maker can subjugate other dragons: their pupils constrict, making them unable to resist the Alpha's commands. This is precisely the power Drago needs to raise an army of dragons.
Swimming. Despite its colossal weight, the Troublemaker is an excellent swimmer, easily outrunning ships and remaining underwater for long periods.
Durable armor. The Troublemaker's scales are among the strongest, able to withstand the combined attacks of multiple dragons.
Here's his version - dark grey, almost gloomy...