The team at Polygon has discovered a way to bypass the original trio and select Pikachu as a starter in Pokémon Go instead.
Instead of selecting Bulbasaur, Charmander, or Squirtle, walk away from the trio to make them disappear. If you are able to repeat this four times, they return with Pikachu. However, as Polygon explained, this can be difficult. Polygon’s full instructions can be found here.
YouTube channel Game Informer has also released a video tutorialon how to achieve this.