Doldrums are dragons mentioned once in How to Steal a Dragon's Sword,
No description is given or imagery seen of this dragon. It is possible Doldrums are a type of larger size Mountain Dragon, as the Razorwings and Tonguetwisters are.
On page 8 (US version) of this book, Doldrums are mentioned:
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.