Navicular is fairly common in older horses especially in the UK as we pretty much have to ride on roads on a daily basis & not all riders are considerate
I also wonder if they removed the shoes as a 'footsore' excuse as if I were selling a horse that was normally shod I'd leave it that way so it wouldnt
risk getting footsore
I can always tell if a horse has navicular by riding it as even on pain meds they don't move exactly right and if I then suspect something I have X rays done prior to purchase - I was often hijacked by local vets doing PPE's for inexperienced riders who don't pick up on it.
There are lots of options now if he's not considered worse case scenario. Correct trimming is essential and therapeutic shoeing can help a lot, barefoot unfortunately isnt often ideal for navicular horses
There are various pain meds but you have to consider long term side effects such as stomach ulcers, liver and kidney damage - my own vet is currently prescribing previcox for long term oral pain relief.
I have known people to have good results with de-nerving but success isnt guaranteed and there are risks. Same as with the use of blood thinners like warfarin which can also be effective
This is a good article that has a lot of facts and new research details Equine Chronicle » Navicular Disease and Treatments
Many UK Mounted Police horses are retired with navicular and still go on to lead long useful lives so I wouldnt give up to easily, if the pony is otherwise what you want then its worth trying to work with the problem