well, if oil becomes unfeasable as a fuel source, you can still use ethanol fueled motorcycles, so horses are still not the most likely form of front line cavalry
you might be able to get robotic type horses, as I recall DARPA is experimenting with a 4 legged robot that can carry equipment, and it is in the late prototype stages. Advance that a bit more and you might be able to make one strong enough to carry a man and armor it against small arms fire
the Robot horse sounds like a waste of resources, but the cycle is our most likely option.