Looks like a sidebone, which is from crappy shoeing/trimming or hard use in the past. It's essentially that the lateral cartilage (which is supposed to be somewhat flexible) has hardedned into bone. It can cause lameness, sometimes not. But is something to have a vetcheck before purchase if you really want to use her for more than a mosey around the pasture. If you can't afford to have a pre-purchase exam, you should probably skip over this horse, as it COULD be something that needs special care, and you can't know that without a vet to check for sure what's going on.
People can dope up a horse to show it to buyers or in certain events, so buyer beware, just because you saw her or were told she is fine, don't take their word for it, take YOUR vet's.