Dragon (animal)

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

Dragon of Monsters (Canon Species)
Oda has mixed the Monsters story into the One Piece main story:

The powers and abilities of a dragon, not only can it destroy a single town (under control of D.R. and Cyrano) and bring terror to the public, killing one would make enough for Ryuma a legend that stories about him was still pass down from generations to another.

Hogback remarked that Ryuma had slain a dragon. This confirms, or at least hints, that dragons do exist or once existed within the One Piece world. However, Roronoa Zoro expressed skepticism toward this.

Sennenryu (Filler Species)
A Sennenryu (千年竜) 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 on the 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.

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 has the power of the Nemu Nemu no Mi (Video Game only) that helps him to erase 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.