The Storm Dragon is mentioned only once in the How to Train Your Dragon Book series, in How to Ride a Dragon's Storm.
No description or image appears for the Storm Dragon. It might be assumed it is large enough to be problematic for a human to handle.
Madguts the Murderous, leader of the Murderous Tribe, does not speak (he speaks by proxy through Gumboil). There are various theories as to why he doesn't talk, including:
|| Some say he lost his tongue wrestling a Storm Dragon with his bare hands.
||— about Madguts the Murderous
|Dragon species of
Dragon Species from the Books