Dragon (animal)

Dragon is an animal in the series. It is a dragon, or a reptile of some sort, capable of flight.

Sennenryu (Filler Species)
A Sennenryu (千年竜), known as a Millennium Dragon in the FUNimation dub, comes from Warship Island's mostly submerged Dragon's Nest. Every 1000 years, they return to the nest to give birth and die. Their bones, when extracted to create an elixir, can grant invincibility, which is what Nelson Royale and Eric desired. They only appear in the anime only Warship Island Arc, and first appeared in Episode 55, but are also referenced in the Ocean's Dream arc. The only named one is Ryu, though a huge group did gather during the Dragon's Nest's rising.

Dragon of Monsters (Canon Species)
Oda has mixed the Monsters story into the One Piece main story. Hogback remarked that Ryuuma had slain a dragon. This at least confirms, or hints that dragons do or once did existence within the One Piece world. However, Roronoa Zoro doubted the existence of dragons and that the samurai killed one.

Tatsunoko Dragon (Filler & Game Species)
A Tatsunoko-ryu (Seahorse Dragon) appears in the Dragon Dream! game, as well as in the anime Ocean's Dream Arc. In the anime, Tatsu, the seahorse, is the main villain who has the power to manipulate minds and memories and also has the power to cast illusions from the memories he gathers. He somehow manipulated the mind of a young boy named Noko who somehow has the power of the Nemu Nemu no Mi that helps him to ease the memories of its victim faster.

Trivia

 * Richie dreamed about fighting a dragon in Buggy's Crew: After the Battle!
 * In Monster Time, Luffy was depicted as a dragon.