And if you were to go back a few more years into the past, you’d have found when you bought a mouse with a PS/2 connector fitted, it may well have come with an adapter for a 9-pin RS232 serial port.

Do you have trouble getting your mouse to work correctly on your Mac? Whether you use Apple's Magic Mouse or a third-party ...

A mouse that plugs into the serial port on a PC. Serial mice used to be the most common type, but were replaced by mice that use the PS/2 and USB ports. Contrast with bus mouse. See mouse.