Thor's Thunderers are gigantic Sea Dragons mentioned in the How to Train Your Dragon Book series.
Thor's Thunderers are depicted as turquoise dragons as big as mountains. They are one size smaller than a Seadragonus Giganticus Maximus, but are as fierce and powerful as one. They stir up the ocean with their gigantic tails. They also have tentacles trailing down their necks and backs, as well as some sprouting from their chins.
Thor's Thunderers are so called because of their ability to produce lightening, and have long "throttling throats" that perhaps sound like thunder, hence their association with Thor, the God of Thunder. They breathe greenish fire that looks like a great thundercloud with flaming electricity. According to The Incomplete Book of Dragons, Thor's Thunders also have excellent vision and project either laser beams or missiles (which is not specified).
Each one of their many tendrils contains a poisonous sting.
- "Three gigantic Thor's Thunderers slowly emerged from beneath the waves and stretched their long necks into the sky, like giant beanstalks growing out of the ocean. Up, up, up those beanstalks grew, long tentacles trailing beautifully from their necks, every one of those tentacles tingling with poison as strong as a man-o'-war jellyfish."
- ―Book 12
As tough and formidable as Thor's Thunderers are, they are still afraid of the mysterious dragon Guardians of Tomorrow.
Thor's Thunderers were seen in the final battle between humans and dragons, on the side of Furious.
Thor's Thunderers are detailed in this Reference book and also appear in the 'Dragon Key' located at the back of the book.