The Name Rater is in Eterna City. It is North of Oreburgh City, and West of Celestic town. Remember that you can only change the names of Pokemon that you have named yourself. Names given by people that you trade with are unchangable.
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In the large building in Eterna City on the first floor. He is the old man. Remember, you can't change the nickname of a traded Pokemon.
Piplup is a penguin Pokemon that first appears in Pokemon Diamond and Pearl. Because Piplup is a penguin, a good nickname would be one based on its penguin counterpart.
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Unfortunately, you cannot nickname traded Pokemon in this game, or any Pokemon game for that matter. You can nickname your own caught Pokemon by going to the Condominiums next to the PokeMart in Eterna City and ask the old man near the entrance. I hope that you will make do!
Go to Eterna City and go into the building, that has the Name Rater on the sign. Talk to the old dude at the table and he will "rate" your pokemon's nickname and ask if you want to change it. You cannot change a traded pokemon's nickname.