Is it known how thick Late Medieval horse plate armor (for field of battle) was? Was it as thick as its rider's armor?
Good horses were expensive and valuable, especially during a battle, so it makes sense to protect them. I would imagine it would be a disaster for a knight if his horse got injured during a fight. He would be very immobile on foot.
As a side note, did longbowmen shoot at the horses? It would make sense if they are less protected.