Well, in medieval warfare, at least according to my sources (people who's into recreating medieaval combat as a sport) horses were very valuable and a knight would prefer to avoid killing it if at all possible. Indeed, they would even avoid killing its rider, since the ransom could make the knight very rich. But, really no point going into that, since in The One Ring, just about every foe the characters meet in combat won't give a toss about chivarly or honour, and no one will be cross with characters attacking an orc's warg mount before the orc.
That said, a horse is a very powerful animal, and they're not fragile at all. That goes double for an armoured horse. Most swings against a warhorse with a sword will only make it mad (even more so if it's in pain).
But, the main goal here is to make additional rules that fits in with a rpg based on Tolkien, so I'm studying how horses are used in the books and in the movies. Another goal here is to have rules that doesn't slow down gameplay, but rather gives it more options and excitement. Making the horse even more easy to kill will defeat this purpose, I think, especially since players will spend many Fellowship Undertakings training it.
Next up, adding barding to horses, to give them a Protection roll when attacks hit them. But how to represent barding being heavy and cumbersome? Hmm...
-Eirik