Don L. Penrod gives an overview of White Horse Prophecy and concludes it should be rejected as apocryphal in light of historical and textual issues.
Don L. Penrod, "Critical Analysis of Certain Apocryphal Reports in the Church of Jesus Christ of Latter-day Saints as Related By Members of the Church," M.A. Thesis (Provo, Utah: Brigham Young University, 1971), 50-62
[Conclusion]
As in all things, the final decision rests in the mind of the individual. The alternatives for a Latter-day Saint seem, however, not to lie in weighing out the wisdom of the world in an attempt to decide if the prophecy is true or false, but in whether or not to follow the sustained authorities of the Church.