The special features for Godzilla 2 indicate that Rodan didn't always have fire powers.
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It appears that Rodan's fiery abilities come as a direct result of hibernating in a volcano.
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In the featurette included on the home video release, "Reimagining Rodan", Godzilla 2 director Mike Dougherty talks about how Rodan's physiology.
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According to Dougherty, Rodan's body adapted to living in volcanic conditions.
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Over time, he grew a volcanic outer shell.
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This means that prior to living in a volcano, Rodan may have been more like his Toho counterpart.
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In classic Toho movies, Rodan does live in volcanoes, but doesn't typically possess fire powers.
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