Doldrums are dragons mentioned once in How to Steal a Dragon's Sword, page 8 (US version):

"They were a mixture of some of the nastiest types of dragons in the Archipelago: Razorwings and Tonguetwisters and Doldrums and Vampire Ghouldeaths."
How to Steal a Dragon's Sword

These were the Red-Rage dragons that attacked Hiccup and the other young vikings as they slept in hammocks on the cliffs at the Gorge of the Thunderbolt of Thor.

No description is given. Its possible Doldrums are a type of larger size Mountain Dragon, as the Razorwings and Tonguetwisters are.