In order to catch the Spiky-eared Pichu, you first need the event Pikachu-colored Pichu that Nintendo released either at U.S. GameStop stores or via Nintendo Wi-Fi Connection. Once you have this specific Shiny Pichu, the Pikachu-colored Pichu, you need to have it as the lead Pokémon in your party and then you have to go up to the shrine in Ilex Forest and then you'll be given the Spiky-eared Pichu. There is no need to catch it.
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I don't think that there is a code for the spiky-eared pichu and only at HG/SS
You can get a Spiky-eared Pichu in Pokémon HeartGold and SoulSilver if you get the movie-event Pikachu-colored Pichu and bring it to Celebi's Shrine in Ilex Forest in HeartGold or SoulSilver.
you need the shiny pichu from the gamestop event. go to the ilex forest shrine with pichu in your party. press A by the shrine and the spiky eared pichu will pop up and join your team if you have an open spot in your party.
The Spiky Eared Pichu from the Event Pichu is holding one. Take the event Pichu in the Ilex Forrest and go to the shrine. The Spiky Eared Pichu will join your party. You can also get one from the S.S. Aqua Captain.
You must get the Pikachu-colored Pichu and transfer it to HG. You will get the Spiky-eared Pichu if you go to the Celebi Shrine in Ilex Forest.