As far as I know, it is impossible for the Spiky Eared Pichu to exist outside of HeartGold and SoulSilver. The Spiky Eared Pichu can only be naturally obtained in HeartGold & SoulSilver, and since those games don't allow you to transfer or trade it, then it would be impossible for it to exist in other games. Besides, HeartGold and SoulSilver are the only games with the sprite for the Spiky Eared Pichu.