Wikia

The Dragon element is a unique, ancient and rare element. Monsters such as Teostra, Fatalis, Alatreon, Stygian Zinogre, Ceadeus and Goldbeard Ceadeus naturally use the Dragon element as a physical weapon, and hunters are able to transfer the strength of a dragon into their weapons to more easily defeat foes who are weak to Dragon, such as Rathalos and Kushala Daora. It is the rarest element found in nature, and in most cases the strongest element to use to defeat monsters. Whenever the hunter hits a monster, a reddish-black lightning orb appears. Strangely, monsters that use the dragon element tend to be weakest to the dragon element. This element is more detectable by looking to a monster's body parts such as Teostra or Fatalis' horns which start white and finish black just as normal dragon-element weapons.