Rotom

Rotom is a very rare dual electric and ghost type Pokémon. It is one of the top five rarest Pokémon in the game.

How to obtain: 

-Rotom used to be in the 2015 event in which you could find it if you had a ghost type Pokémon and a electric type Pokémon in your party.

-You can obtain it through the roulette.

-If you have an electric type Pokémon and a ghost type Pokémon in your party, you might find it. It has been emcountered before in Mysterious Grotto in the grass in the cave with the ladder. (This method of finding it has not yet been comforted)