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Early on the shadowy tail led me to think Moltres was gonna be Ghost/Fire. I figured they were going to do an anti-phoenix approach. Phoenixes die and come back, so they're kinda like a ghosts or undead in that way. The purple-pink flames would also be explained by Ghost Typing since the presence of ghosts traditionally turns nearby flames blue or purple.
I thought it being Ghost/Fire and Articuno being Psychic/Ice would be a clever way of explaining how they fly without being Flying Type. They both levitate with ghostly/psychic powers. Zapdos would be Fighting/Electric or Dark/Electric and the idea would be it doesn't fly but uses electricity to run fast.
In the end I'm satisfied with the Typings we got. They're a clever way of doing "freeze, thunder, fire" without literally using the most obvious types for it.