The Horses Lookup List has been enhanced to include a "Scheduled" / Next Appointment column, and you can sort on this column as well as on the Due Date column.
These new features are available in both the full-screen and mobile sides.
Please see this topic for more information and a tutorial on using these features to determine which horses are due when, as well as which horses haven't been scheduled yet:
http://www.backroadsdata.com/iFAforums/viewtopic.php?f=17&t=175