My riding horse is 22 years old, and I had compliments ALL the time of people asking to buy him and saying he looks like a 3-yr old. He was well-muscled up and had no weight problems whatsoever.
Then, 3 years ago, his best friend Bo since he was 3 foundered and passed away. So we had to find a friend for Arthur real quick so he wouldn't be lonely (we only had 2 horses then....the other few horses wed had had passed away from old age by then, and Arthur and Bo were the last from the crop my parents bought when they first moved here 18 years ago. We ended up rescueing a mini mare who was tied to a fence her entire life, and the owner stated without shame that the mare had been tied to that same fence her entire life and "didnt have a problem with it." The other horses were living in really small chicken wire pens....but she said she was selling most of them so I hope she followed through. Anyway, Arthur hated the mare but at least it was something. Completely ignored each other. Then, just last spring we bought a Paint Clydesdale gelding and a Welsh stally from a guy who was having a kidney transplant and couldn't take care of them anymore. Arthur, being dominant over everyone, started herding Lulu (the mini) around incessantly so no one else could get to her. It was insane. He lost so much weight, and we didn't know what to do so I went to a bunch of equine nutrition seminars and finally found a diet that worked for him. Now he looks like the 3-yr old again, at age 22.
There is (most of the time) a solution....they probably just don't know what it is yet. Or it IS animal neglect, in which case I'm going to rip my hair out waiting for the SPCA to call me back!