The PlayStation Mouse (SCPH-1090) is an input device for the PlayStation console that allows the player to use a mouse as a method of control in compatible games. The mouse was released in Japan on December 3, 1994, the launch date of the PlayStation.
The mouse itself is a simple two-button ball mouse that plugs directly into the PlayStation controller port without adapters or conversions and is a fully supported Sony accessory. It was packaged along with a mouse mat bearing the PlayStation logo.
Source: Wikipedia, "Playstation mouse", available under the CC-BY-SA License.