Early computer mice attached to workstations with various proprietary protocols and plugs. Mice for home computers began with serial connections (RS-232) with DB-25 plugs, as for Logitech's C7 series. Their P7 series were parallel with pins instead of socket plugs, judging from a blonde one at oldmouse.com.
With a Logitech bus mouse card a mouse could connect to the specific DE-9 socket or the other style round D-ring bus socket, each on an internal card designed for the mouse. Microsoft developed its In-Port™ mouse card for the round bus plug.
The wide serial plugs eventually downsized to DE-9 plugs and then to the small round PS/2 over time. Mouses using both of those connectors ran with their own internal microprocessor. An array of adapters followed. Manufacturers developed adapters to change the plug to enable using a mouse on different computer configurations rather than having to produce identical mouses but with different style plugs. At Logitech, Mike Lavelle designed the one piece DB-25 to DE-9 adapter that other manufacturers imitated shamelessly. For that audacious flattery he received a 1990 award at the end of the year from Logitech.
Lavelle DB-25 socket to DE-9 adapter (500066) and matching Logitech mouse plug (500068).
Knock-off copy of Lavelle's DB-25 socket to DE-9 adapter that came on a Z-nix mouse.
Twenty-some years later the Logitech adapter still feels nice to grip with a tactile rubbery feel and well defined ridges on the thumb screws. In contrast, the plastic on the cheap knockoff is hard and slippery making it less efficient to grip the screws.
DB-25 hole socket
Logitech mouse PS/2 to DE-9 socket adapter to use a PS/2 mouse on a computer with a serial port. Diagrams courtesy of Mobius at Wikipedia.
Serial and specific mouse ports gave way to the standardized USB Connection for most peripherals on all manner of computers.