If I level up my Groudon in Gen III, then transfer it to gen 5, if i take it to the move relearner, will it know earth power, or no?
Well yes, Groudon learns Earth-power at level 65 in Gen V so if you get him higher than that yes. If you transfer a Pokemon from a previous game then it will learn the moves that it can learn in the new game at the levels in the new Game. Also when you transfer a Pokemon with a move that has a different amount of PP the PP changes as well.
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