catch a dragon Pokemon such as horsea and Bagon until you get one holding a dragon scale.
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you need to give seadra a dragon scale (which you can obtain by catching or using thief on some dragon Pokemon (bagon for instance) and theres a chance that they will be holding a dragon scale) and then you trade your seadra holding the dragon scale and voila you have a kingdra
you have to catch a dragon Pokemon holding a dragon scale
Dragon vs dragon is super effective :D
To get a dragon scale is to catch a horsea or a bagon.
You can evolve Seadra by trading it with the Dragon Scale