The XF86Config file tells the X server what kind of monitor, video card and mouse you have. You must create it to tell the server what specific hardware you have. The interface type can be determined by looking at the connector of the mouse. The USB mouse has a flat rectangular connector.
Some mice come with adapters with which the connector can be converted to another.
You can attach multiple USB mice to your system or to your USB hub. There may be more than one serial port to which the serial mouse can be attached. The next step is to guess the appropriate protocol type for the mouse. In FreeBSD 2.2.6 or later, the X server may be able to automatically determine the appropriate protocol type, unless your mouse is of a relatively old model.