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This was explained by Morimoto Shigeki years ago regarding Lugia
>To start, the first type was decided to be Flying-type since Lugia have wings. At the time Psychic-type was the symbol of powerful Pokemon. As we wanted Lugia to give the impression of being a powerful Pokemon, we settled on Psychic-type for the second type, instead of Water-type.
>In short, the reason why Lugia is not a Water-type despite being underwater, is due to its Flying-type-like appearance, and Psychic-type being the symbol of a powerful Pokemon.
That's why there are so many Psychic-type legendaries; Psychic type is simply symbolic of how powerful they are
