@Polk and Alex
Most of the times I'm being shot at, I'm not usually riding towards the archer, usually perpendicular or away from the archer. A not so strong hit won't do much damage to the horse, like 1/4. A medium hit will do 1/3 to 1/2. A strong hit with a large speed bonus will naturally do a lot anyways, around 60-75%. My horse has rarely been 1 shot except against exceptional arbalest users, and obviously some STR-focused melee. 4-5 was a bit extreme on rouncey anways since I see 5 as a maximum not an average. It's closer to 3-4.
Then there are attacks that hit you instead of your horse, taking up a variable number of hits. How important this is depends on how well that character is geared towards melee.
So anyways, I'm still in favor of making the head more of a weakspot and removing the legs thing. Making horses slower or whatever will only be a pain and just cause people to get off. The head is slightly harder to hit, and not many people will have access to the head unlike the legs if the cav is stopped by melee.