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Speed Evolutions
Speed Evolutions work like this: you have a Pokémon whose final evolves really late. (Bagon, Dratini, Larvitar, etc.) But with a "Speed Stone", you can evolve your Pokémon immediately into a "Final Pokémon". It won't be as powerful as the Pokémon final evolution though.
Some Pokémon will also lose something that will be included in their finals. For example, Bagon and Dratini will never have flying type. They may get diffrent types, like Bagon will get Dragon/Ground and Dratini will get Dragon/Normal(or Fairy)
