Horses have just recently been added in the snapshot. There are two unused textures, a skeletal horse and a zombie horse. I don't know what the zombie horse would be used for, but I think the skeleton horse should be ridden by a type of skeleton. These skeletons would wear at least a leather hat and would have a stone sword or an iron sword. They would basically be a rare mob that would ride around you, making passes every so often.
Riding these skeleton horses would be cool as well, what if you could kill the skeleton on it and steal the horse?
.... What, do you expect to get fame from this suggestion? I doubt they would put in the texture files and not do anything with them. Besides that, you need to give them time. They probably already have this in their special development version, but they have yet to implement it into snapshots.
Not sure. Kind of stupid riding a 'skeletal horse', because when the sun comes out, they'd burn to a crisp. A zombie horse would also be ... not smart.
The horses haven't even come out yet and people are already making conditions. Heck, we don't even know what they're going to act like, yet. They might add a cart, skeletal horse, zombie horse and more. Perhaps waiting to see what's actually in the update, first...