The Thunderhead Tribe, also known as the Thunderhead Bay Vikings, were a tribe of Viking whalers that lived on Thunderhead Bay, Northwest of Berk.
The very small tribe of the Thunderhead Bay Vikings appear to rely heavily on the sea for their livelihood, as opposed to a more agricultural-based Tribe. In "The Eternal Night", Fishlegs indicates they primarily rely on whaling, but most likely they fish and harvest other marine creatures as well to eat and possibly trade. They also appear to have a knowledge of sea dragons or Tidal Class dragons, since they were able to successfully capture and utilize Scauldrons.
According to Gobber and Stoick, the Hairy Hooligan Tribe and the Thunderhead Tribe were involved in an event called the "Valhalla Wars". The cause of this conflict is not mentioned, but ended when the Thunderheads "had to sue for peace".
During the events of "The Eternal Night", the Thunderheads seek revenge in a novel way. The chief, Snorre the Vast, picks up a self-proclaimed sorceress to trick Berk into believing they had a curse laid on them. It was timed during a solar eclipse so that it appeared that she had caused an unnatural darkness. While the Hooligans were dealing with that, the Thunderheads stealthily kidnapped various Hooligans and took them prisoner. It appeared as if they 'disappeared' by magic.