Kur.
Kur, in Sumerian mythology, is considered to be both first ever dragon and likely the firstborn monster of Tiamat herself. The name 'Kur' is also usually referred to the Zagros mountains to the east of Sumer.
Description[]
Although Kur was sometimes the home of the dead, it is possible that the flames on escaping gas plumes in parts of the Zagros mountains would have given those mountains a meaning not entirely consistent with the primary meaning of mountains and an abode of a god. The underworld Kur is the void space between the primeval sea (Abzu) and the earth (Ma).
Overview[]
In the Enūma Eliš of Akkadian tablets from the first millennium BC, Kur is part of the retinue of Tiamat, and seems to be a serpentine dragon. He was the first dragon to emerge with it also being implied that Níðhöggr was his "twin" sibling as they both share the same characteristics of being death dragons.
In one story, the slaying of the great serpent Kur results in the flooding of the earth but an undiscovered version, one revealed by John Constantine, shows that once Kur was slain by Marduk he fashioned the underworld Irkalla from his corpse much like how he made the heavenly abode of the Mesopotamian gods from Tiamat's. Níðhöggr was spared but imprisoned by Marduk beneath the roots of the World Tree.