You have to give it a dragon scale and trade it.
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Wait, are you, by any chance, talking about the Water/Dragon Pokemon, Kingdra? If you are, then you are correct. In order to Evolve your Seadra into a Kingdra, you must have him hold a Dragon Scale, and then you must Trade the Pokemon to a friend. However, this is the questions &answers section. This where Pokemon Fire Red players who need help with the game come and ask questions. But, it looks to me like you were trying to answer a question. If you wish to do that, then click on the question you want to answer, and then you will see a small box below the question. Use that box to type your answer. After you are finished, click the small checkbox besides submission terms, and then click submit your answer!
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Hope this helps.
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