The Mount window is used to display your horse armor and barding/saddlebag information. It will also show items in your horse's saddlebags.
The window will appear when you click on the button labeled "Mount" in the Status Attribute window of the User Interface. In addition you can type the command "/mount" to activate the window. You can click and drag the active horse icon image to your quickbar and then use it to quickly mount and dismount from your horse.
There is nothing shown when the window is concealed. The window is invisible with no graphics or controls to minimize the amount of memory used. Nothing will appear on the screen when you attempt to activate the window.