Brigham Young teaches that a "prophet" is a "foreteller of future events" without requiring priesthood; "Apostle" is one who holds the Keys of the Holy Priesthood.
Brigham Young, "The Lord's Supper—Miracles and Manifestations of the Power of God—the Gospel and the Gifts and Blessings Thereof," Journal of Discourses, 26 vols. (Liverpool: Horace S. Eldredge, 1871), 13:144
Many persons think if they see a Prophet they see one possessing all the keys of the Kingdom of God on the earth. This is not so; many persons have prophesied without having any Priesthood on them at all . . . To be a prophet is simply to be a foreteller of future events; but an Apostle of the Lord Jesus Christ has the keys of the Holy Priesthood, and the power thereof is sealed upon his head, and by this he is authorized to proclaim the truth to the people, and if they receive it, well; if not, the sin be upon their own heads.