One of Piloswine's basic attacks is Ancient Power. If yours doesn't have it them you probably evolved it from Swinub and he has not learned it. You will need to take this pokemon to a move tutor so that he can learn the attack. Once he has learned it, next level up should evolve it to Mamoswine.
This is what Marriland says: "(Use a Heart Scale on Piloswine after it evolves to get it to learn AncientPower, because it does not learn it naturally.)"