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I don’t think dual types were an idea until most of the Kanto dex had been designed. We know bird type was initially there. I think every Normal/Flying type was pure Bird and every other flying type only had their other element, which would explain why Charizard couldn’t learn fly in Red and Green. I’m not sure what caused the decision to make dual types, maybe the legendary birds? But I do know like 90% of the Kanto dual types don’t look like they need two types. Every Rock/Ground type could be just Rock. Most Grass/Poison types don’t look outwardly poisonous save Vileplume. Cloyster, Jynx, Gengar, the Nidos, Lauras, etc don’t look like they need two types.
