Also known as "Monster Rancher" in the west, this series was a short-lived rival to Pokemon, that spun out of the late 90s. It's main gimmick was that you would collect the monsters, using real life, physical media; like inserting a different CD into your system, and then would contain a random monster! Obviously you can't do that with a DS though, but maybe you can put in... like... your credit card or something.
Please note the game is entirely in Japanese, but can be played on any Nintendo DS system.
