Gloomers are not seen in the book series, but are discussed in the Complete Book of Dragons. They are featured in the Bog Dragons chapter, but are shown in the Mountain Dragon section of the Dragon Key at the back of the book.
Gloomers very much resemble Eeyore from Winnie the Pooh in appearance and attitude.
Gloomers are depicted as being a uniform baby or sky blue, with a droopy tail, droopy wings, droopy spines along their back, and long, droopy ears.
Despite their sad dispositions, Gloomers do breed and lay eggs. Their eggs "are as dark and as heavy as lead", and are difficult to lift even freshly laid. The eggs become even more weighty as they incubate. This will cause a depression in the ground that becomes deeper and deeper until it hatches, placing the baby at the bottom of a dreary hole.
Despite depression, Gloomers are intelligent, as they are capable of speaking Dragonese.
Gloomers are called what they are because behaviorally they are nearly, perpetually, depressed. They Cheering up a Gloomer can be quite a task.
Gloomers are used as Driving Dragons by the Vikings.
Gloomers also sided with the Humans during the Dragon Rebellion.
Gloomers are detailed in this reference book and also included in the Dragon Key at the back of the book. They are also classified conflictingly as both Bog dragons and Mountain dragons.