Equestrian: Horse Tricks
A collection of things Altaïr can do on a horse in Assassin's Creed.
Jump mounts: You have to land on the horse from a height that you would otherwise survive. The third one (from the fence) is a 16-bar stunt mount, meaning Altaïr would lose 16-bars of sync if the horse weren't there to break his fall. Any higher and he falls *through* the horse and dies. UPDATE: TheGuyOnTheScreen demonstrated a successful jump mount from a fatal height. I have replicated the mount from a View Point exactly as shown in his video and believe that it would work at other locations in the Kingdom. My initial conclusion must have been faulty. More study is needed to determine why Altaïr sometimes falls through the horse.
Dismounts: You can aim your dismount to the front, side or back. Sometimes you can dismount advantageously: into a haystack to Vanish, onto a View Point, or over a guard post.
Changing horses midstream: Aim your dismount from one horse so that you mount another horse.
Invincible rearing: The horse can take unlimited abuse while rearing. There's a brief window between rears when it's vulnerable to being knocked down. Meanwhile, the guards kill each other (two or three die in this short clip) especially in Memory Block 6 when their swords cause the most damage. Credit to ianxofour for this concept.
Fighting on the horse: Everyone knows how to do this - hold Right Trigger and press X (R1 + Square on PlayStation 3).
Fighting behind the horse: The horse will kick anyone who stands behind it during a fight, even Altaïr. It throws guards off balance but doesn't cause any permanent damage.
Fighting inside the horse: A glitch that happened on the way to Arsuf. I was fighting on the horse and somehow fell inside of it (that's when I started recording). The game thought I was still on the horse. None of the controls worked and neither of us could move. I had to quit the game - and it was a Replay of Memory Block 6 so all progress was lost. 
Take the high road: an interesting route through Lakeside Village on the western edge of the Kingdom. As far as the flag in the ruins, it can also be collected on foot if you approach from the other side. Only the horse can make the jump from this side.