Monster Rancher Battle Card GB (モンスターファームバトルカードGB) was released in North America on December 23, 1999, for the Nintendo Game Boy Color system. It is the first game in Tecmo's Monster Rancher Battle Card series, featuring a card game incorporating the popular characters from the Monster Rancher series. It was followed by Monster Rancher Battle Card: Episode II for the Sony PlayStation entertainment system which featured additional monsters.
Although the Monster Rancher series is known for the ability to import game content via CDs, Monster Rancher Battle Card GB has no equivalent functionality. Instead, the player takes a short "personality test" to determine his starting deck and then can win additional cards by defeating opponents as well as new monster cards by defeating dungeon bosses. While a code system is also in place, only a select few cards catering to a single monster along with some general support can be obtained through this method.
Source: Wikipedia, "Monster Rancher Battle Card Game", available under the CC-BY-SA License.