Those look like foot spurs- kind of like a callus on a human foot. Some pigs get them, others don't, even living in the same environment. They aren't harmful, the only risk is that they could potentially catch on something and tear into the soft part of the foot, which would hurt. You can trim them with nail clippers, but careful not to go too close to the foot- kind of like a hangnail, the ends are old dead skin but they are attached to normal skin where they meet the foot and you don't want to injure that!