This is a 23yo mare I used to know. When this photo was taken, she was on daily phenylbutazone, had bad laminitis (and still getting molasses in her feed!), also had cushings (the cresty neck and hairyness), and was being ridden five days a week. Not mostly walking and a little trotting, but full blown lessons with cantering and jumping. Corrective shoeing and pain relief can only do so much, and Jane was a saint when all things are considered.
I regret riding this mare, but at the time I didn't know any better. I know better now, and I wanted to give people a reminder of how bad horses can get, and how greedy some instructors can be. I'm not saying you should ask to see a health certificate or anything, but if something doesn't feel right, just say no. From riding another horse with a funky trot due to ringbone, I failed to notice when my first horse had an abcess in his hoof. I still feel guilty.
She also probably had buttercup poisoning because of the paddock she was kept in. She was euthanized in 2007, and still being ridden a week before the vet call, at approximately 23 years old.
Despite all her problems, and being a grumpy old hag at times, this mare would have made a very cute paddock mate - she loved a good scratch on her cushings lump - and I'd almost guarantee her attitude would have improved if she wasn't being ridden on sore feet.