I imagine a mount slot item as something the mount carries/wears.
When you use the Figurines such as the ebony fly they tend to be of use in just one or two combats--with a standard action they grow to full size and you can choose to already be mounted, but what happens to the mount slot item? Is it automatically "mounted" or do you have to get off and take a few turns placing it on your mount in the middle of combat?