You can make any equine game halloween-y
Mounted games could include
"candy dash" - a lot like the western flag race have the rider run to the other end of the arena grab a handful of candy and run back, judge by fastest time, and give a bonus prize for the person who brought back the most candy.
"egg an spoon" - do a rail class holding a hard boiled egg on a spoon, if it falls your out. Great for all ages!
"ride a bat" you can get those cheap rubber bats give one to each rider have them sit on it again do a rail class... Posting trot..... Who ever keeps the bat wins!
Of course you can do a costume contest have riders dress up themselves and there horses!
I've got more ideas if you need more.... These are the easiest ones for both English and western riders and riders of all ages :)