Ice Age Horses

The ancestry of the modern horse is a bit confusing issue to me, but on the other hand it seems to be that for experts too. The Przewalski horse, or the Mongolian wild horse, is pretty much straight forward, it even looks like the prehistorical horse images on the walls in many caves. But why are modern domestic horses so different? Okay, a chihuahua doesn't exactly look like a wolf, being a striking evidence of how selective breeding can change a species over a relatively short period of time. But what disturbs me most about the modern horse is the length of the manes of all domestic horses. Is it just a result of selection done by humans? In which case I wonder why it has been seen as something so much more preferable than a short mane? Are all modern breeds of domestic horses descendants of one single lineage in which a long mane had become apparent through selective breeding? Or was there a type of ancient wild horse, perhaps the (probably) hypothetical "forrest horse", which lived apart from the other horse types, maybe in more humid weather conditions, where rain and snowfall occurred more often, giving a horse with a longer mane some advantages against the shorter standing variety, which is standard in the wild extant equine species like zebras and asses and even the Mongolian wild horse? Of course, all illustrations depicting Ice Age horses or any other extinct species (or subspecies, or types) are hypothetical, so it doesn't matter so much how I paint them really, it's the context that is interesting. And it's not my task to invent the theories, my job is to make visual interpretations of them. But sometimes, like in this case, it seems quite hard to find even a good theory of the origins of a specific feature.