How do I kill the guards that are on horses easily?

Question

I always whack away at a mounted guard, but it seems I only hit the horse. When I'm done, the guard is still chilling on the horse.

What's the best way to defeat these mounted guards?

Answer 1

One of the 'loading tip' says that when they come towards you, you have to be equipped with a one-hand sword and hold R1 (PS3 version). You'll block the charge and put the horse down. It's easy from there to execute them when they're still down.
Another easy way is to use pistol, crossbow or (spoiler below, hover your mouse)

even less work by calling your assassins friends when your brotherhood is created.

Answer 2

Shoot 'em. They die/fall off instantly. Plus in Brotherhood you don't even have to change to that weapon, just select and hold down attack to shoot. Crossbow is better than pistol except in this regard.