Rodan wasn’t the second monster out of the Toho stables (that was Anguirus from the second Godzilla movie), but he was the first Toho Monster seen in color, and more importantly, the second Toho monster introduced in his own film. One reason that I think Ghidrah the Three Headed Monster works so well is that Godzilla, Rodan and Mothra were all appearing together for the first time after starring in their own films, creating much more of a sense of a shared universe than if they had all been introduced in the Godzilla series. Then add in Ghidrah, a monster so powerful that it took all three Earth monsters to defeat him. (The inevitable decline of Ghidrah began with Monster Zero, when it was established that Godzilla and Rodan alone could whip him. They should have stuck with all three monsters.)
