A mouse by the British company Advanced Memory Systems.
The mouse was connected to the joystick port of the CPC. An additional lead plugged into the power port in the monitor where the disc drive gained it's power. The disc drive power lead could plug into the AMX Mouse power port.
There were three versions in all.
Other ways to connect a mouse to the CPC
Pictures
Software
AMX Art Stop Press MAX AMX 3D Zilicon