Several months ago, I shared my dissatisfaction with the D&D hit point mechanic, and compared it to the Lord Of The Rings: Strategy Battle Game approach. LOTR:SBG uses a combination of wound and fate points instead of hit points. Wounds represent physical damage, while Fate represents your ability to avoid a wound, dodge or parry a blow, or otherwise escape injury. While those two types of "damage pools" each operate a little differently in LOTR:SBG, I feel that the similar approach could be used in D&D.
The D&D rules for cavalry, horses, and mounted combat are similarly dissatisfying. They are dissatisfying because there are no rules in D&D for mounted combat! Having spent the last 45 minutes trying to locate something in the way of mounted combat rules, in the AD&D books, I finally turned to Chainmail.
Chainmail provides some guidance in regards combat between mounted and foot units. In the Chainmail rules, 2 light footmen attacking 1 light horseman have a 16% chance of killing the horseman. Conversely, 1 light horseman attacking 1 light footman has a 45% chance of killing the footman. A medium horseman has an even better chance of killing a light footman, somewhere in the 65% range.
I like the way LOTR:SBG handles combat between cavalry and footmen. In LOTR:SBG each rank-and-file figure has one attack. However, any mounted figure gets an additional attack, if charging. If the mounted figure wins the attack, while charging, he gets twice as many chances to wound the footman. Therefore, since the horseman had two attacks while charging, he gets double that (4 chances) to wound the footman. Conversely, if a footman wins a combat against a cavalry figure, there is a 50% chance that the attack will hit the horse instead of the rider.
I think similar rules could be used in D&D. You could give an attacker on horseback an extra to-hit roll. That attacker could roll all of his attacks at the same time. If the attacks hit, you could then double the number of damage dice rolled.