ok, im just gona through this out there...
I know this post is old. But don't make posts unless you have researched ur information. You gave a completely false answer to someone. I am not here to say the 360 controller will work on the original xbox... But apparently you have no idea how the xbox works. The original xbox has full usb functionality. M$ just striving to be different, made a custom port to try and fool people, guess it worked, fooled you.
The xbox is just a low-mid(in its time) end computer. know with that understanding. the 360 controller and regular controller are both USB gamepads. Now I am willing to bet that the xbox will not recignize the 360 controller as a xbox 1 controller, NOTE: I do not know this as a fact. If it does not, I am sure the driver can be ported for the 360 controller and loaded into memory via a custom dashboard.
Now is it work all the modding to get a 360 controller to work on the original xbox? You decide!
I am going to atempt to get my wireless controller to work on my xbox with the pc reciver module avaliable from M$, If it works sweet, if not... then maybe I'll mod the dash myself. I'll keep you posted .
It's an intresting project to me.