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 of 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")
In 2009, a dragon was one of the creatures created by the citizens of Admah for their gladiatorial fights. Dr. Rodney McKay was sent out to fight the dragon wearing armor from Admah, wielding a lance and riding a mechanical horse, all of which he could control due to having the ATA gene. As the battle went on and McKay figured out how to use the armor, he realized that the weapons he'd been given were specifically geared towards defeating the dragon and his helmet's interface revealed that it was not the first dragon to fight and die in the arena. Though this particular dragon was the "big daddy" of all the dragons to fight in the arena, the information the helmet provided on how the other dragons were defeated provided McKay with a roadmap of what to do. McKay was able to hit the dragon in three different vital places that killed it, but the dragon swung its monstrous tail even as it died. The blow flung McKay and the mechanical horse into the arena wall so hard that it crumbled, presumably killing him. However, McKay was secretly wearing an Ancient personal shield and survived the blow, although he was briefly knocked unconscious. (SGA: "Brimstone")