Rotom

About
Rotom is a Pokémon often confused as a Legendary or a Mythical; as in this game, it's not a regular Pokémon. You find this Pokémon on-field by summon.

Where is Rotom? Where are its forms?
Below is a chart that will help you out here. And the items are found inside the houses of the town with the Lake Guardian statues.

Trivia

 * Rotom is affected by half EXP rate as well; despite not being a Legendary or a Mythical.