The Elemental Dragons are mystical creatures that live in Ninjago. They are said to be connected to both Ninjago and the Underworld, enabling them to travel between the two worlds at will. They were the Guardians of the four Golden Weapons, and are generally not vicious unless the Weapons are threatened.
Kinds of Dragons
Ninjago has 8 known Dragons: the Fire Dragon, the Lightning Dragon, the Ice Dragon, the Earth Dragon, the Golden Dragon, The Overlord (his original form was a dragon), the Ultra Dragon, the Nindroid MechDragon, the Green Dragon, and most recently, the Titanium Dragon.
Sensei Wu placed each Dragon to guard one of the Golden Weapons, each in tune with its elemental properties. The dragons guarded the weapons for many years, until an increase in Skulkin activity led Sensei Wu to believe that the weapons had to be retrieved before they could fall into Lord Garmadon's hands.
The four Ninja had to avoid each of the Dragons as they recovered the four Golden Weapons. When the Weapons were stolen by Garmadon, they discovered that the Dragons only wanted to protect the Weapons, and would gladly aid the Ninja in their quest. Afterwards each Ninja kept the Dragon matching their elemental properties as a pet, in addition to using them for transportation. This allowed the Dragons to continue guarding the Weapons, as the four Ninja wielded them in defense of Ninjago. As Jay decided to visit his parents, all four Dragons were molting and had to leave the Ninja. The Ninja had to go on the journey to Ed & Edna's Scrap N Junk on foot.
During their time in the Spirit Coves, the Dragons morphed into the Ultra Dragon. It appeared during the fight against the Great Devourer and nearly defeated the creature. It briefly served as Lloyd's personal transportation until he mastered the Golden Dragon, most notably in "Ninjaball Run".