There have been many different versions of Pikachu over the years. He's been an artist, a detective, a rock star, and even a ghost Pokémon (sort of). None of those manage to trump the electric mouse dressed up as Nintendo's Mario. It's so fucking adorable.
The crossover is the theme for a new limited range of merchandise, created thanks to a partnership between The Pokémon Company and Nintendo (the company that partially owns the Pokémon Company). The merch will include cuddly toys, keyrings, cards, notebooks, 3DS accessories, t-shirts, and more.
Unfortunately they're looking like Japanese exclusives for the time being, so you'll probably have to pay a serious amount of cash to get your hands on this adorable little bastard: