Kelly, the horse with the dipped back is probably just terribly old.....they go that way if they are lucky enough to live that long.
A note of caution.....a few years ago I saw a horse in a field that looked depressed, moth-eaten and skeletal. I was outraged and called the RSPCA. When I told them the address the Inspector sighed and told me that the pony was well known to them due to the number of complaints they received about him. he was in fact 37 years old, a well-loved family member with no teeth and Cushings who was in fact very well looked after. I felt chastened and told myself off for jumping in with my big feet