Bruce McConkie, in Mormon Doctrine, writes that the White Horse Prophecy is a false and deceptive document and not a true prophecy of Joseph Smith's.
Bruce R. McConkie, Mormon Doctrine, 2d ed. (Salt Lake City: Bookcraft, 1966), 835
From time to time, accounts of various supposed visions, revelations, and prophecies are spread forth by and among the Latter-day Saints, who should know better than to believe or spread such false information. One of these false and deceptive documents that has cropped up again and again for over a century is the so-called White Horse Prophecy. This supposed prophecy purports to be a long and detailed account by the Prophet Joseph Smith concerning the wars, turmoils, and difficulties which should exist in the last days.