You need to get the vet out to see if the horse was drugged and to figure out what where the lameness originates.
It could be anything from in the hoof to tendons to shoulders to somewhere in the hocks or hips, all depending which end of the horse is lame.
Unless a person is specifically taking a horse on to "let it live out its years as a pasture pet", this is why thorough PPE's have become an evil necessit; that means X-rays included.
It used to be that wasn't the case but it seems like the bigger the horse owning population becomes, so goes the "wasn't lame before I bought it" issue
Being pigeon-toed can cause lameness as the horse ages because it affects the bones and tendons in the legs. I am acquainted with a lady who put a horse down for that reason. No amount of money, proper trimming, injections, IRAP, you-name-it could help the horse but, again, it was either late teens or early 20's by that time. She had only owned the horse around seven years so the malformations, thus serious arthritis, had had plenty of time to take hold before she bought the horse.
Good luck to you - I hope whatever "it" is, isn't serious:)