Oh I absolutely love Halloween horse shows! There are so many things you can do, I’ll make a list for you.
Horse is dr.seuss and rider is thing 1/2
Mickey and Minnie Mouse
Horse as Starbucks drink, rider as barista
Nurse and patient
Depending on your horses color... giraffe and safari tour guide
Bride and groom (or bride/bride groom/groom)
Dalmatian, Cruella Devil
Race car and race car driver
Dorothy and the lion
I hope this helps give you an idea! Good luck at your show!!