A big, green, fire snortin' lizard. No, not that one.
Dragons are large reptilian, fire-breathing monsters. Though on modern-day Earth, they are considered mythological creatures by native humans, real specimens have been known to exist.
Dragons can be bipedal, quadrupedal, winged, have necks of various size, and have heads or tails in ranging numbers. Indeed, since the term "dragon" is so generic, it seems possible to describe creatures not native to Earth which fall under the very forgiving criteria as dragons.
Fiction
Generation 1 animated continuity
At least one full-on dragon (reptilian, fire-breathing, winged) has been seen on Earth in medieval times, and a similar but less imposing creature (supposedly from Borneo) in the modern era. The medieval dragon was tremendous in size, dwarfing both humans and averaged-sized Transformers. Although quite dangerous, it was driven off by a wizard using dragon's bane.
Lord Chumley spent years patiently stalking the Template:Tooltip of Borneo before transporting it back to his estate. He later set it against the captive Optimus Prime. Kinabalu was a little larger than Prime, had no wings, and did not breathe fire. It attacked him with teeth and claws, but Prime threw it off a bridge.
Also, the fictional movie monster Hojoni bears a striking resemblance to the medieval dragon, though wingless.
The only known instance of a non-Terran dragon was in the dimension of Menonia, where a wizard called the Golden One could use magic to take on the form of a large, flying golden dragon.
Classics toyline
Also in the G1 continuity is the Mini-Con, Nightscream. He takes on the form of a dragon similar to that of the aforementioned medieval dragon, but red. It might be added that his connector is on his aft.
Beast Wars
The auditions for Reign Of Fire were going well.
When the Predacon Megatron took on the spark of his Decepticon namesake and was submersed in lava, his form mutated from a Tyrannosaurus rex to a large, red, fire-breathing dragon, possibly hinting at an evolutionary connection between dinosaurs and dragons.
Beast Wars II
Puff the magic dragon, shot you in the face
Galvatron had the computer on Gaia (future Earth) download the most powerful creature from the planet's past, which resulted in the DNA of a dragon being downloaded into his system along with the planets most powerful weapon, which turned out to be a Drill-tank for some odd reason, resulting in a partly cybernetic, partly organic beast mode. This dragon may confirm the breed's existence as a species.
Robots in Disguise
Among the various forms used by the six-changer Predacon Leader Megatron, a two-headed dragon was one of his more well-known (and more creative) forms. Later, after his conversion into the more powerful "ten-changer" Galvatron, he gained a second (one-headed) dragon form. (This was originally a pteranodon in Car Robots, but was changed for the US release.)
Unicron Trilogy
The leader of Jungle Planet, Scourge, transformed into a mechanical dragon with the ability to deploy up to three heads. Though the Jungle Planet is populated entirely by Transformers who transform into mechanical beast modes seemingly modeled after Earth creatures, it is unknown if Scourge's alt-mode has any direct connection to mythological dragons of Earth, which he expressed confusion about.