It has been confirmed by the Pokémon Company that the costume is not a Pokémon, just a generic green monster.
I Quote: "As seen in the Pokémon video games, Poké Maniacs are Trainers who love dressing up as intimidating Pokémon and other creatures. In costume as a Green Monster Poké Maniac, this is a special Pikachu plush—it's not even dressed as a Pokémon at all! A removable hood and big claws are ready for romping and stomping."
Source 2, if Source 1 stops working one day.
May this quench your curiosity.