I recently watched a western that showed some (feasibly only one) extremely sweat-lathered horses. They'd been ridden hard, but I honestly can't remember if it was in the context of a horse-race, outrunning Indians, or whatever. I'm kinda stuck because I've already tried to reference it in an answer over at english.se What does "Whip into a lather" mean?
Sorry - I know that's not much to go on. But the lather was pretty extreme!