We had a young mare acting similarly (5 yrs old at the time) and she ended up having ring bone and side bone in one foot. We knew she had issues there, as she had an old injury from some "not very smart" "person" barrel racing her flat out as a 2yr old. She has one foot that slightly twists in and wears differently. X-rays showed pretty advanced low ring bone and a bad case of side bone to boot. She either needs $2,500 worth of surgery or pasture rest for 3-5 years until the bones fuse on their own. Her owner doesn't mind boarding a pasture pet, so she's going to let her sit.
The horse is sound enough at the walk and will even gallop around with the others ocassionally, but limps badly at the trot and doesn't want anyone on her back. Can't say I blame her . We have her on Smart TLC supplement and keep her thin to reduce pressure on the joint. Her feet are trimmed every 3-4 weeks to keep them balanced, so they won't wear unevenly. We tried specialized shoes/pads, but they didn't seem to help very much.
Good luck with Chili!