An ancient monster that lived in the Jurassic period. Gudon enjoys fighting and attacks using his whip-like arms and sharp fangs.

Gudon appeared before the MAT Gyro that came to investigate tremors occurring at the No 2 quarry on the Tama River, but retreated underground after the MAT Gyro attacked. It then caused an earthquake near a farm. This revealed the egg of Twintail, the Ancient Monster. After Twintail hatched from the egg, Gudon emerged from underground and the two monsters fought. When Ultraman Jack arrived, Gudon attacked him too.

Later, Gudon appeared at Yumeno Island. It was caught in an explosion from MN bombs stashed in the MAT Arrows. The attack had no effect and Gudon went back underground. Eventually Twintail appeared again and Gudon followed. They caught Ultraman Jack in a joint attack, so MAT disorientated the monsters with a sleeping gas bomb. With no sense of direction Twintail attacked Gudon by mistake, and Gudon retaliated by biting Twintail's neck, ending its life. In the end, Gudon is killed in an explosion from Jack’s Spacium Beam.