It would simply demonstrate, with evidence, how both lines developed from a common ancestor and it would have been quite a powerful story that is fairly easily explained.
I wasn't expecting it to change all creationist's minds - particularly the ones found on these boards.
I seem to remember that the horse has a pretty good fossil record - anyone know what its nearest living relative and common ancestor are? ["Hello Google"]
The nearest living relatives of horses are zebras and asses. They're very close relatives, all classed in the same genus, and horses are believed to have split from the rest only about 2.5 million years ago. There is an extensive fossil record of the horse/ass/zebra genus
Equus, with dozens of extinct species being named.
However, despite this, there is nothing we can point to as
the common ancestors of horses and asses/zebras, and there never will be. The simple question is, how could we possibly ever know? Think back to crashfrog's analogy of family relationships. You and your cousin share a common ancestor, your grandfather. Imagine you then come across pictures of your grandfather, your grandfather's brother, and your grandfather's cousin. All of these may share some distinct family traits, and in all of them you might be able to see the resemblance. Without some sort of documentation, however, you would never be able to tell which one was your grandfather, or even if none of them are and your grandfather's picture was lost.
And that's similar to the situation we're looking at in fossils. We can identify the family relationships. You can dig up a load of fossils of horses and their relatives, and you can tell that they're all relatives. If the material is good enough, it's possible to make pretty good family trees. But there will never be a way of distinguishing between a (metaphorical) grandfather, grandfather's brother and grandfather's cousin.
Having said all that, if you want the scientists' best guess as to the fossil nearest to the horse/ass/zebra ancestor; the answer is
Equus simplicidens from Idaho about 3 million years ago.
Edited by caffeine, : spelling and messed up tags