I was riding through the pine woods yesterday and thought, "wouldn't my horse look cool with some mail or plate armor on"...
Why bother? Horses are a burden in the game. I've never had one for more than one adventure because the stupid things haven't the sense not to tangle with cave bears and sabre cats.
They also look silly (the animations are pitiful) and are ridiculously slow.
If Bethesda releases horse armor, I hope someone finds a way to beat them to death with it.