Princess Celestia, one of the rulers of Equestria and she has been that for over three thousand years, she even ruled for one thousand years alone. One thing she never had and that’s a vacation but after seeing her sister in the Hoenn league, she decided to have her own vacation/journey in the human world, her own Pokémon vacation/journey.
This story takes place at the same time as Luna's journey in Kalos, you don't have to read Luna's story to read this, Celestia's story is it's own story. Did that make any sense at all? There might be some spoilers for Luna's story.
Mlp:Fim/Pokémon crossover, I own nothing.