Two different formes of Pichu appear in Pokemon Movie 12: Arceus and the Jewel of Life. The first is the Pikachu Colored Pichu, and the second is the Spiky-Ear Pichu. The Pikachu Colored Pichu is a shiny Pichu, and the Spiky-Ear Pichu is a Pichu with one Spiky ear. The Spiky-Ear Pichu is unable to evolve.
When the Spiky-Ear Pichu is brought over to HGSS from DPPt, an in-game event happens. You team up with the Pichu and go into the Ilex Forest to find the Pikachu-Colored Pichu. When you finally get to the Ilex Shrine, both Pichu battle you in a double battle.
Spiky-Ear Pichu's Moveset:
Pokeball: Cherish Ball