Rodan

Rodan (ラドン Radon) is a giant Pteranodon kaiju that first appeared in the 1956 Toho film Rodan, and debuted in the Godzilla series in the 1964 film Ghidorah, the Three-Headed Monster.

Origins
Rodan is a Pteranodon that survived in an egg underneath Mount Aso for 65 million years. Radiation from nuclear testing seeped into the cavern containing Rodan's egg, and in combination with rising global temperatures and accumulated volcanic gases caused it to hatch once it was unearthed. The hatchling grew quickly by feeding on Meganulon in the cavern before flying out. A second Rodan also appeared around the same time, and mated with the other. It is never clarified exactly how Rodan became gigantic, as characters note that the creature is far larger than any Pteranodon on record. While nuclear radiation is believed to be responsible for awakening Rodan, it is never proposed that the radiation was responsible for causing it to reach gigantic size, at least not officially yet.