Probably not really close to as much as 500 pounds. The problem was, to really armor a horse in an effective way is difficult and expensive. It meant plating or chain-mailing the body of the horse, which would have been extremely heavy when added to the weight of the knight and his gear. So typically what you would see was, if the thing was armored at all, it would have a sort of helmet to protect the head, occasionally a piece (a "crinet") to protect the neck, and, rarely, a breastplate (the "peytrel") to protect the chest as well. Most of them made do with just a chamfron-style helmet for the horse, though - like
this one. All told, that's really not that much more weight to do it that way.
There were monstrosities like this ca. 1450 Italian piece, but realistically, they were hugely expensive, and beyond the reach of most knights - having such a thing would be very much a status symbol, and would have been pretty rare. Besides, by the time full-blown horse armor like that was being deployed, the heavy mounted knight was, again, on his way to extinction anyway.
Agreed. By less than 500 lbs, I meant that even the most extreme extravegant use of arms, armor, tack, and decoration it is certainly a lot less than 1000 lbs., illustrating and supporting medved's point...
