Go to Lavender Town, the one with the Pokemon Tower. There should be a house where you can change your Pokemon's name. There should be a sign near the house saying Name-Rater.
you go to Lavender Town and look for a house that says Name-Rater
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You can rename a pokemon by going to the Name rater
Yes you can trade leafgreen version Pokemon to another leafgreen version.
On Pokemon Pearl's title screen, if the Pokemon LeafGreen game is in the GBA slot, click on "Transfer From LeafGreen" and you can choose six Pokemon from LeafGreen to transfer to Pearl, but be careful-you can't trade them back.
Manaphy is a legendary water type of Pokemon. It is not possible to get this Pokemon in the game Pokemon LeafGreen.
Piplup does not exist in Pokemon LeafGreen or FireRed.