Thanks again to Starry Might in our forums and also to Bulbapedia for bringing this up.

If you live in the U.S., you'll be able to pick up your very own event shiny Pichu from a participating GameStop from January 30th through February 14th. This is the very Pichu that unlocks the Spiky-Eared Pichu (known as the "Notched-Ear Pichu in the Japanese versions) in HeartGold and SoulSilver. Check out the HG/SS Special Pokemon page for more info on the event that unlocks Spiky-Eared Pichu.