Using Expression Interactive Designer things like creating a listbox which scrolls horizontally become really easy. Below I've detailed the steps of achieving this by editing the listbox's ItemsPanel Template (ItemsPanel template determines the panel that will control layout for listbox items).

Steps:

1.  Create a listbox from the library palette
   
2. Add a couple of items to the listbox, in EID you can easily do this by right clicking on the listbox and selecting the "AddListboxItem" menu option
    Note: so far the listbox looks, and scrolls like any traditional listbox
   
3. From the Format menu select Other templates | ItemsPanel and choose to make a copy of the item template
    Note: you are now editing the panel that will hold the items in the listbox and be responsible for the layout of the listboxitems
4. You'll notice that the ItemsPanel template contains a VirtualizingStackPanel, select it and from the properties palette change orientation to be Horizontal
5. Scope up to the root (using the timeline scope up button) to finish editing the template

At this point you are done your listbox now contains a custom ItemsPanel that holds items horizontally instead of vertically.