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That's the let down about FireRed and LeafGreen. They cut off all connection to the Pokémon from other regions like they don't exist - which clearly they do if some of them even try to evolve during game play such as Golbat into Crobat for example. I'm afraid the only way you're going to be able to trade Kingdra back is to obtain the National Dex. :\
I will say though, that if you're interested in talking more about Third Generation games like FRLG you can do so in the Advance Generation Games (GBA) forum we have available. This thread here isn't an introduction unfortunately.