This is far from the only time Monster Octopus' art style has been used in a Square Enix game. The Japan-only cell phone game, Kingdom Hearts Mobile first used these sprites across its collection of minigames back in 2008. The DS remake of a different cell phone game, Kingdom Hearts Re:coded, allowed players to make their own avatars in this style as well. Most recently, only a few weeks before Theatrhythm's release, Square Enix launched the Monster Octopus' designed Final Fantasy Brigade on cell phones across Japan.
Over the last few years the art style has become popular enough to warrant more than a little merchandise. An ever-growing collection of cell phone straps (think key chains for your phone) are available on the Japanese Square Enix Store website along with other choice items.
Now if only they would make a phone strap of everyone's favorite giant purple octopus.