During the Age of Ancients, the Dragons were the everlasting rulers of the world until the Lords rose and challenged them. In the end, the Dragons were nearly - but not fully - driven to extinction, beginning a new era known as the Age of Fire.
Age of Ancients
Not much is known about the world at this time; only that the world was shrouded by fog, filled with grey crags and arch-trees, and that the immortal Everlasting Dragons held dominion over the land.
War of Fire / "Dragon War"
With the birth of the First Flame - the Advent of Fire - the Lords challenged the Dragons for dominion of the world. During the war, Gwyn, the Lord of Sunlight used his powerful bolts, peeling their stone scales; the Witch of Izalith and her Daughters of Chaos wove great firestorms; Nito, the First of the Dead unleashed a miasma of death and disease; and Seath the Scaleless, who sought the immortal scales of the dragons, betrayed his own kind.
Age of Fire
In the aftermath of the war, The Age of Fire came about, bringing an age of sunlight and prosperity. While the Everlasting Dragons appear to be all but extinct, the race has many descendants throughout the world, some greater and some lesser, with varying degrees of similarity to their progenitors.
From what can be glimpsed in the opening, the Everlasting Dragons were towering beings with a skin covered both by stone scales and bristly dark fur. Their head sported two straight horns and a mouth bearing two rows of sharp teeth on both jaws. Their back ended in a long and heavy tail but they were still able to fly thanks to their wings, leathery and clawed like the ones of a bat. Appropriately for creatures that lived in a world of fog and penumbra, their two eyes had vertical slotted pupils (this kind of eye is the symbol of both the Path of the Dragon and the Dragon Remnants). Aside from these reptilian features, the Everlasting Dragons' body was a lot less beastly than that of their descendant and kin: their arms didn't end in claws but in four-digited prehensile hands while their torso, broader at the shoulders and thinner at the hips, was almost humanoid in appearance.
Seath and the Stone Dragon prove that these beings were capable of higher reasoning and could understand other species' language to at least some degree, via their known contributions to the world (the former, Seath, being the founder of soul sorcery).
The Dragons' iconic characteristic, their stone scales, was implied to have a strong connection with their own form, based on the description of the Dragon Scale item. The exact truth of this, however, is undetermined.
Throughout the series as a whole, draconian qualities and covenants are seen by several characters or are mentioned in item descriptions as being believed to be a way for individuals to transcend their current form.
It is still a mystery if the Everlasting Dragons' destructive breath is true fire (as suggested by the Dragon Head Stone) or a flame with arcane properties (like Kalameet's); however Kalameet's flames are at least partially Dark in regards to game mechanics.
The Hellkite Dragon encountered in Undead Burg is actually a Drake. Drakes are seen as undeveloped imitators of the dragons, but are likely their distant kin. Wyverns may also be of similar origin to the Hellkite Dragon and are found in the Valley of Drakes.
In Ash Lake, the Stone Dragon, a descendant of the ancient dragons, initiates the player into the Path of the Dragon Covenant. Domhnall of Zena shares a rumor that the Stone Dragon is not a descendant and is instead, a surviving Ancient Dragon.
Undead Dragons, scattered throughout the land, are corpses of the Ancient Dragons that maintain some spark of life within their rotten form and can be occasionally stumbled upon as minibosses.
Dark Souls II
The Guardian Dragon residing in a cage in Aldia's Keep, is one of Aldia's creations and guards the way to the Dragon Aerie which is filled with the Dragon's kin. Despite its name, this creature is a wyvern (it has two legs and two wings) and its features are both avian and draconic. Being a creation of Aldia, it is possible that the latter interbred the last surviving drakes with some giant avian creatures in order to create a self-sustaining new race of Dragons.
The Ancient Dragon residing in the Dragon Shrine is said by the Emerald Herald to have watched over the world for "aeons". However, the veracity of this is doubtful, and he may have been resurrected by Lord Aldia than a survivor from the War of Fire, since Aldia attempted to recreate the dragons using giant souls. The former is far more likely as when the Ancient Dragon is slain, the Bearer of the Curse receives a Giant Soul.
Imperfects are eyeless horrors with a giant humanoid mouth and the ability to breath dark spheres. While their appearance is far-removed from a dragon, they drop dragon-related items. The meaning of this is unknown.
Dark Souls III
Very few true Dragons appear in Dark Souls III, if any at all; as even more time separates the events of Dark Souls III and the Age of Dragons. Wyverns appear in Lothric Castle and the High Wall of Lothric and two Ancient Wyverns reside in Archdragon Peak. However, the Wyverns in Lothric Castle are also afflicted with Pus of Man.
Oceiros, the Consumed King was formerly the King of Lothric but became obsessed with Seath the Scaleless' work and sought to become more powerful using his royal blood. He discovered research made by Logan, who in turn was inspired by Seath. Much like Logan, Oceiros was driven insane by the work of the paledrake and somehow transformed into a malformed abomination reminiscent to that of a dragon.
Archdragon Peak is a location accessed through the 'path of the dragon' gesture found behind the location where Oceiros is fought. The enemies there are adherents to the Path of the Dragon, most likely based on the covenant from Dark Souls. Various sizes of these man-serpent hybrids appear, wielding various weapons such as axes on chains, daggers and spears. The Ancient Wyvern is also fought here.
Company Captain Yorshka seems to be a dragon crossbreed similar to Priscilla. She has scale-like markings around her eyes, and a tail not unlike that of a dragon's.
The Wretches found in Irithyll dungeon are a twisted hybrid of a human and dragon who were left in the dungeon to rot, possibly due to being failed experiments.
The Nameless King is referenced to have sided with the Dragons and is speculated to be the first-born son of Gwyn, indicated by several descriptions on weapons and his boss soul. He is referenced as a dragon-slaying god of war who sacrificed everything to side with the Dragons, and Gwyn's first-born son is said to have been stripped of his stature as punishment for his foolishness and effectively removed from history as a result of that. When encountered, he is riding a Stormdrake, named King of the Storm.
Darkeater Midir is the only remaining archdragon encountered in Dark Souls III who is still alive. Raised by the Gods of Lordran themselves to combat the growing Abyss, Midir's everlasting battle against the darkness has taken its toll on him, and he is slowly being consumed by the dark's influence. Fearing that the darkness within him may drive him mad, the Ashen One is tasked by Shira to put him down before he loses himself.
- According to an interview with Miyazaki, the ancient dragons are half-living, half-element, and do not feel pain. The dragon depicted in the opening cutscene pre-dates the emergence of life in the world.
- Souls of Lords description.
- Opening narration.
- Drake Sword description.
- Dragon King Greataxe description.
- Dragon Greatsword description.
- Domhnall of Zena dialogue.
- Soul of Priscilla description.
- Bequeathed Lord Soul Shard description.
- Obsidian Greatsword description.
- Calamity Ring description.
- Game no Shokutaku Interview