Rodan is a kaiju.
Rodan was a giant Pteranodon mutated by radiation from atomic bomb tests. His egg survived into the 20th century in suspended animation within a huge cavern near the dormant volcano of Japan's Mt. Toya. Eventually hatching, the young Rodan instantly devoured the entire population of Meganulons, which had been menacing the nearby town of Kitamatsu. Soon growing to adulthood and possessing a wingspan of roughly 500 feet, Rodan and another of his species (whose origin was likely the same as his own) attacked the city of Sasebo and were eventually done in by the combined efforts of the Japanese Self-Defense Force and the suddenly-erupting Mt. Toya. However at least one of the Rodans survived and went on to become a staunch ally of Godzilla.