Dragons are supposedly mythological creatures found in a number of Earth mythologies. They are capable of breathing fire, and some are even capable of flight.
As one the guardians designed to protect the Sangraal, Morgan Le Fay used a dragon that appeared when one attempted to take the hologram of the Sangraal. It was said that to speak the guardians name would cause the dragon to stop. The dragon could have been real, although by the time it was made the Ancients had developed solid-hologram technology. Even throwing C-4 down its throat did nothing to stop it. Dr. Daniel Jackson then realized that the guardian was not the dragon, but Morgan Le Fay, who created the dragon and the tests. Vala Mal Doran ran up to the dragon and yelled "Morgan Le Fay", but nothing happened. Daniel then yelled Ganos Lal, Morgan's name in the Ancient language. The dragon then disappeared with a flash which supports the idea of it being a solid hologram. (SG1: "The Quest, Part 1", "The Quest, Part 2")