What an interesting premise! It's actually the idea behind a cute horse book that I just finished, from author Jackie Anton. Called Frosty and the Nightstalker, it takes place in both the 1900s and the 1800s, with human and horse characters from each time period.
Frosty is a young appaloosa horse who begins to have a very bad dream when under a tranquilizer, given by a veterinarian for an operation. Later, as he goes on trail rides with his owner, he has difficulty at water crossings. More than normal difficulty, it seems.
Many horse people make jokes about their horses spooking at tiny things, chiding their mounts: "what, did you just see a ghost or something?" But in Frosty's case, every time he crosses a river or stream, the memories come deep and strong - as if transporting him to another place and time.
Which leads us to the other part of this story, set in the 1800's, where a horse named Nightstalker, who belongs to a herd of appaloosas owned by the Nez Perce tribe, is about to experience changes and adventures he could not possibly foresee.
Author Jackie Anton does a good job here, weaving elements of both history and horse knowledge into a very interesting concept for a horse book. With lovely illustrations by Sandy Shipley, there is much information and lore about horses in general, and appaloosa horses in particular, that a young horse lover might enjoy.
Frosty and the Nightstalker is actually Book 2 of the author's series called Backyard Horse Tales. You can find links to buy them, as well as excerpts, at Jackie Anton's website: http://talesbyjackie.com/Jackie/Welcome.html
And for you horse people, next time you ride a horse that spooks at something, you might wonder what is really going on in his mind!