Digital Monster Ver. WonderSwan is a Japanese handheld version of the original Digimon pet for the WonderSwan. It includes all of the original Digimon from the first four virtual pet devices and the first pendulum device. In this game the player can have up to eight different Digimon with them at a time. It has computer-controlled opponents to battle with. The game has the ability to connect to another WonderSwan through a special link cable. It can also hook up to the original pets through a Digimon "dock N rock" connector.
Source: Wikipedia, "[en.wikipedia.org/wiki/Digital_Monster_%28virtual_pet%29%23WonderSwan Digital Monster (virtual pet)]", available under the CC-BY-SA License.