Dragon

Dragons were reptilian creatures from Earth myths and legends. They resembled giant lizards, often with wings and/or the ability to breathe fire. In 705 there were dragons in Krakow. (PROSE: Only Human) According to some sources, much like the ancient Egyptian gods, dragons appeared to be also be a native species to Earth, sometimes serving as omnipotent beings. (COMIC: Vortex Butterflies, etc.)

Fiction or reality?
It was commonly believed that the classic markings of "Here There Be Dragons" on maps were metaphors and that dragons weren't real. However, when someone said as much in their vicinity, the Gestalt stated that this was simply "what the dragons want you to think". (PROSE: Ring Theory)

During his confrontation with Light, the Seventh Doctor mentioned dragons, among other imaginary creatures, which Light had failed to catalog. (TV: Ghost Light)

Of Earth origin
In the Middle Ages, the First Doctor encountered a fire-breathing dragon in the forest outside the Pied Piper's magical castle, but drove it off with a fire extinguisher. (COMIC: Challenge of the Piper)

The Ninth Doctor recounted slaying a dragon in Krakow. (PROSE: Only Human)

Torchwood Three once dealt with a water dragon in Cardiff Bay. (AUDIO: Torchwood_cascade_CDRIP.tor)

Dragons were found on the planet Avalon, having been brought to life by the system of nanobots which had long ago been devised by the Cephlie. In the 22nd century, human colonists arrived on the planet and the nanobots, which acted on mental impulses, brought to life the fantasies of those colonists, including that of dragons. One such dragon chased down the First Doctor, Susan, Barbara and Ian, before it was slain by Sir Bron of Westhold. (PROSE: The Sorcerer's Apprentice)

Elsewhere
A dragon was the guardian of the Vault of Plenty on Gemino. It killed anyone who failed to provide the correct answer to the puzzles required to gain access to the Vault. (COMIC: The Secret of Gemino)

River Song once transported dragon eggs on the Starship Harmony and Redemption. (TV: The Husbands of River Song)

The Twelfth Doctor once claimed, when being attacked by a race of dragon-like beasts who were indigenous to a planet, that dragons were fictional. Lord Mortigan explained that the dragons had to have been genetically modified "a little" by the human colonists. They were inhibited and enslaved by the colonists' inhibitor chip, which the Dragon Lord had freed them from. The dragons subsequently killed the royal family as well as the Dragon Lord in the Red Castle. The dragons were left to roam the planet freely, after the Doctor contacted group of rescue ships to pick up the surviving humans. (COMIC: The Dragon Lord)

One Dragon was trapped in the the Void. (COMIC: The Infinite Corridor, Old Girl)

estimated human pioneers would hunt Mexxonian Dragons to extinction in 2543. (PROSE: The Dragon of Hyacinth Lodge)

In culture
Chinese dragons were used in dances on board the Monarch's ship. (TV: Four to Doomsday)

Harry Sullivan remembered a Disney film based on Arthurian legend, in which one woman transformed into a pink dragon. (PROSE: Wolfsbane)

Imitations
Senator Alcott's fear of the Chinese manifested itself as a legendary Chinese dragon-demon. (TV: The Mind of Evil)

The Dragon of Iceworld was a biomechanoid assigned to keep the Dragonfire away from Kane. (TV: Dragonfire)

Dragons existed in the dreamrealm inhabited by Avalon. The Fair Folk used them to harass infantry and prevent UNIT's domination of the air through the use of jet fighters. The Eighth Doctor rode one deep into Fair Folk occupied territory in order to attempt to forge a ceasefire. (PROSE: The Shadows of Avalon)

A golden dragon statue stood in the House of the Dragon, Magnus Greel's fortress. It could be used as a weapon, firing powerful beams from its eyes. (TV: The Talons of Weng-Chiang)

When the Timewyrm took full possession of Chad Boyle's body, she reshaped it into the form of a small dragon. (PROSE: Timewyrm: Revelation)

The Bestiary of Legendary and Magical Creatures created a dragon out of human and animal body parts. (PROSE: Phoenix)

On one world, Enamour was produced by a mechanical dragon. (PROSE: The King's Dragon)

As epithet
Humans labelled the Draconians as Dragons, an unflattering nickname. (TV: Frontier in Space)

As food
On Tír na n-Óg, Daffyr drank mead while enjoying a feast of dragon meat. (PROSE: Cat's Cradle: Witch Mark)