Of course everybody knows Dreamworks fantastic dragon Toothless. From How to Train Your Dragon, Toothless brings a funny and laughable tone to the movie. Being a friendly dragon, (unlike others I could name) he works together with the main character. Now if dragons were real you would never be able to tame one like you see in the movie. Toothless breaths or shoots plasma. Now fire would be explainable with a gas and a spark in the mouth it would work but plasma is to unexplainable. 

The Hydra from Greek myth is a nine headed serpent. A dragon with poisonous blood and breathe. This dragon is slain by Heracles for one of his 12 labors. The famous trait of the hydra: every time a head is cut off 2 grow back in its place. This is not actual true with the Greek hydra, for when you cut of its head one grows back in its place. Heracles defeated the unbeatable enemy by cutting off 8 of its heads then burning the stumps, then crushing the main head under a rock.

"The last of the great dragons of Middle Earth and the primary antagonist in J. R. R. Tolkien's The Hobbit." This terrifying Beast is from The Hobbit or The Lord of the Rings, Smaug. These series of movies have given rise to D&D or Dungeons and Dragons. Smaug has a great pile of gold stolen from the dwarves. He invades the dwarves kingdom. Thirteen dwarves, the wizard Gandalf and Bilbo Baggins go to take the kingdom back. In the book they describe Smaug as "a most specially greedy, strong and wicked worm". He is one of the strongest dragons in the world J. R. R. Tolkien made.