At local or non-rated shows, the class specs will change from show to show or at least vary between the different organizing bodies that govern them. To know the specific requirements of the classes you named there, you'll need to check the prize list for the show.
That said, the same types of names are usually used nation-wide, so I can make an educated guess that "future hunters" is a hunter class set at a low height, probably under 2'6, but almost certainly under 3'. It is also possibly restricted to horses/riders with limited experience.
Short Stirrup pleasure is likely a class aimed at younger riders (at A/AA shows SS classes are for kids under 12) that takes place on the flat and judges the horse's manners, way of going, etc.
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