Its in pearl and Diamond, After you get national dex go to pokemon manor south of Hearthome City, If you look in the room to the left of the carpet inside the manor you should meet Mr Baclot, Now you may want to save before talking to him so you can change it if you dont like the choice, If you talk to him and tell him you are envious he will claim to have a certain pokemon in his Trophy Garden to the rear of ther manor. The pokemon he claims is there is normaly a normal type but they are quite good ones like Porygon, Jiggalypuff, Castform and so on :)
Problem is the new pokemon only lasts for one day and you will have to search a while for it. Not to mention the chance of getting a Male Jiggalypuff is very small as they are mainly Female :)