These are the expenses I can think of just now.
Fencing - all fencing needs regular maintenance
Yard Grooming Equipment - for manure removal, landscaping, managing weeds, dealing with water erosion, moving hay, etc.
Water - troughs / waterers need maintenance and replacement from time to time, also water supply to keep dusty areas watered down, etc is an expense.
Insurance - of course!
Pest Control - don't know if you have destructive pack-rats in your area, or any nasty insects, but they may need to be managed.
Shelters - horses tend to eat them or rub on them!