World Dragons

The World Dragons (世界竜 Sekai-ryū) refer to the nine creatures that reside in the World Tree (世界樹 Sekai-ju) Yggdrasil. While none are actually dragons, they became known as such because of their draconic appearances, as well as their immense powers.

They each guard a Mythical Zoan-class Devil Fruit, which have become known as Dragon Fruits.

Dragon Fruit
The Dragon Fruit (竜の実 Tatsu no Mi) is a special type of Devil Fruit that is each guarded by a World Dragon. It is a Mythical Zoan-class Devil Fruit which, unlike other Devil Fruits, respawns at the Yggdrasil rather than reincarnates itself into the nearest fruit upon the user's death.

Nidhogg
Níðhöggr, anglicized as Nidhogg (ニーズヘッグ Nīzuheggu), is a wyrm-like creature with a hundred eyes on its body. It resides at the bottom of Yggdrasil, guarding the Tatsu Tatsu no Mi, Model: Wyrm and devouring the corpses of the impure.

Trivia

 * It's supposed to be based on the World Tree Yggdrasil and Norse cosmology, but it's really confusing and still work in progress.