From the Wikipedia entry: "List of Ultraman Monsters":
"First Appearance: Episode 25 'Rogue Comet Cyphon'"
"Gigas (or Gigass) was 39 meters tall and weighed 15,000 tons, with a design and character very similar to that of the legendary yeti. Gigas was a natural creature who lived in the mountains of Japan until the comet Cyphon (Tsuiphon) pass close towards Earth's orbit, causing the giant to reveal himself."
"At the same time and location, the Science Patrol were searching for six missing nuclear weapons that could accidentally go off. After being chased by Gigas, they then encountered the comet's own native monster Dorako. The two monsters soon met with one another and fought for hours until a third beast, the violent Red King, who unfortunately has also eaten the said missing bombs, appears and easily defeats them both. After Red King kills Dorako by tearing off his wings, Gigas fled the scene, only to be destroyed soon afterwards by a new weapon developed by the Science Patrol."
Via The Vault of Buncheness