If real horses were known to the Nephites, one problem is that we would expect them to show up in more of the art in Mesoamerica. Johnson points to one possibility, but many questions remain.
On the other hand, Brant Gardner in his thorough new book, The Book of Mormon as History (Salt Lake: Greg Kofford Books, 2015), explores horses in the Book of Mormon as a potential issue of translation to English (pp. 295-300), arising from Joseph's personal assumptions about the context rather than a direct translation (this, of course, would favor the "loose translation" approach to the Book of Mormon, at least for unfamiliar flora, fauna, and objects). His treatment of the word "chariot" is especially insightful, linking it to Mesoamerican litters.