Thunderer

The Thunderer dragon was mentioned toward the end of "How to Seize a Dragon's Jewel", as a member of Furious' Dragon Army. It is not clear if this is a different species, or the author meant to refer to a Thor's Thunderer. Not much information is given about this Thunderer, other than it is presumably quite large, a type of Sea Dragon, and can speak Dragonese. "Thunderer" does not seem to be a proper name, as it is prefaced with "the".

"The great Sea-Dragon Thunderer came joyfully to tell the news to Furious. ... So the Thunderer was surprised to find Furious in such abject misery"