The trick is, it is an engraving on a watch. Clues are: watch made in 1891, American Waltham (made in Boston), owned by a man who was a Methodist preacher in the Tennessee area. Family legend says he may have been aided by the Estes family of Tennessee. All of these may provide some clue as to what kind of horse. But most importantly, the mane is crenellated. And if not stylized, the eyes are huge. And this is also a portrait. Not sure if this suggests that this is an individual horse remembered in the engraving, or it's just a horse. In researching crenellated manes, the only thing I have found is the Norwegian Fjord Horse, but there is some doubt there would have been any in pre-1891 Tennessee.
Thanks for any info you may be able to give me. This is my great-grandfather's watch; it is more a romantic curiosity than anything else.