When you think of "Godzilla vs. Megalon", you think of Megalon, Gigan, and Jet Jaguar, but there are some other monsters that appeared as well. If you blink, you'll miss them, but they are there. Alot of it was stock footage, but there was new footage filmed of one monster... Angilas. At the very beginning the "so called" nuclear blast goes off and causes the monsters on Monster Island to go nuts. It's not really a nuclear blast but stock footage from 1968's "Destroy All Monsters", when the Killak aliens release gas that turns the monsters on mankind and causes them to attack all the major cities on Earth. All Toho did was tint the film red and tacked it in the begging of the film. Well, during that scene 2 monsters appear... Angilas and Rodan are shown. Rodan was total stock footage, but Angilas was shown with stock footage AND new footage.
Rodan...
Angilas...