Dragon (animal)

Dragons are an animal species in One Piece. They are large and mythical reptiles capable of flight.

Dragon of Monsters
Oda has mixed the Monsters story into the One Piece main story:

The powers and abilities of a dragon are so strong that not only can it destroy a single town (under the control of D.R. and Cyrano) and bring terror to the public, but killing one made Ryuma a legend, and stories about him are still passed down after generations.

Hogback remarked that Ryuma had slain a dragon, however Roronoa Zoro expressed skepticism toward this statement.

Dragon of Punk Hazard
The Straw Hat Pirates encounter a dragon on the first island they visit in the New World, Punk Hazard. It is capable of speech.

Sennenryu
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
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.