Traded Pokemon act different; you can't change their name, they get more experience etc. In the old days, if I recall correctly, trading once meant that pokemon was always "traded", even if your trainer ID matched the pokemon's owner ID.
Is that still the case in Fourth Gen? I've got some pokemon in Diamond I sort of want to give to HeartGold but I'm wondering if they'd be considered "traded" forever.
Best Answer
As long as your name and trainer ID match that of the given pokemen, it will not count as a traded pokemon. This means it's possible (but very unlikely) that you could trade for a pokemon and have it not count as traded.
Even if you name all your characters the same thing in each game, however, you're still overwhelmingly likely to have different trainer IDs, so it will count as traded if it's not on the original cartridge.