Dragons live in the caves of Mongo. They are curious and apparently hungry. However, they can be distracted from their prey by the arrival of a rival lizard.
Flash Gordon and Dale Arden are menaced by a dragon (which is simply called a "monster") in the second and third installments of the Flash Gordon (comic strip) comic strip. ("On the Planet Mongo") Flash tries to fight the monster with his clasp-knife, but the creature's hide is too strong for the blade to penetrate. Luckily, the monster is distracted by the arrival of another beast, which looks something like an orange brontosaurs.
The blood of dragons is poisonous, and is used in ritual battles. ("The Monsters of Mongo")
Flash Gordon, Dale Arden, and Dr. Hans Zarkov are menaced by dragons in Chapter 101: The Planet of Peril of the 1936 Flash Gordon (serial) serial. The trio escape when the dragons are killed by a ray gun blast from Officer Torch's rocket ship. In Chapter 102: The Tunnel of Terror, Flash and Princess Aura are saved from being tossed into a pit of dragons.