Mount&Blade is most similar to Pirates!, except it is set on land and has huge, real-time battles with each fighter having his own AI process; the original game allows up to 100 combatants, but the Battle Size Changer tool can raise this as far as 1000.
No; the armoured horses actually have more than double the protection of the unarmoured ones, so a lot of the time you can die before your horse will. To offset this the tougher a horse is the slower its top speed is. Rather than having an armoured version of each horse breed (which is unrealistic) there are instead special breeds which come with the armour already attached (and are much more expensive). There are also modifiers which determine bonuses or penalties any given horse has, so there is some variety within each breed, and this greatly affects the purchase price. Horses are expensive and ones with the best modifiers are fairly rare, so a favourite horse falling in battle can be quite a serious blow.