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, or even another Seadragonus Giganticus Maximus. 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".

"ATTA-A-A-A-A-A CCCKKKKKKK!!!!!!!" roared the dragon Furious to his second-in-command, a great Sea-Dragon called the Thunderer."
How to Seize a Dragon's Jewel
"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."
How to Seize a Dragon's Jewel