Dragons are a race of huge, winged, fire-breathing beasts (they take offense to being compared with lesser reptiles). The youngest specimens are at least twice the size of a grown man, while the largest can swallow men and horses whole. They are highly intelligent, and are adept spell-casters (though they obviously possess a natural affinity for fire and heat related magics).
The largest and most powerful of all dragon-kind was a massive beast known as 'the Eldest', with a body so massive he was forced to crawl on his belly. He was a favored servant or pet of Azzathra, and was used by Keeper Alphel to devastate Ami's dungeon. She tricked the dragon into putting his paw in the pool of a newly made temple (which seemed to suck the very life from the beast).
When Mercury took control of Malleus's dungeon, one of the minions to join her was a young dragon. The Dragon showed no fear of her at a later date, although it's unclear if it joined in the first place because it was cowed or because it was too much of a pain to move and search out new employment. Cathy used him to train Ami for the duel against the Horned Reaper, and he often behaved like a huge cat.
When Ami decided to leave Malleus's dungeon and felt it necessary to let most of her employees go, the Dragon was one of those who did not come back when she tried to rehire them later on. He did however return when she abandoned the Avatar Isles for her dungeon in the mountains, after hearing that she had attained great wealth and wishing to make his home in her treasury.
Crowned Death is also known to keep zombie or skeletal dragons among his forces.