The first is the ability to shoot another person's horse directly with a BANG! Indirect attacks like Gatling or Indians! cannot target the horse. In response, the horse-riding player can try to avoid the attack in the usual ways (Missed!, Dodge, Barrel, etc.). If it gets hit, he loses the horse (no last second beer for your horse). I think this is fun, but I wouldn't allow you to directly target the horse at any distance. I think that only if the player is seen for one more distance beyond your reach then you can target his horse with a BANG!
Of course, there is the problem of horses being less useful with this house rule. There are some ways to remedy this. Perhaps the horse could have 2 health. The suggestion that the Dice Hate Me people give is for the horse to die, but it becomes transformed into a Barrel. Turn the Horse sideways and now you can hide behind its carcass! The dead horse can be panicked and discarded just as a Barrel would. If you already have a Barrel, the horse doesn't stack, but is discarded.
Their next idea is to allow Hideouts to be blown up. So the horses can be shot, and Hideouts can be destroyed with dynamite. If a player has dynamite in his hand, he can play at like a brown card on another player's Hideout to immediately destroy (discard) the Hideout. I don't know if I like that it is immediately discarded, as you can avoid the attacks with the horses. A good balance might be for the player to "draw!" If black, his Hideout is destroyed. Otherwise, it is fine.
In general, I like house rules that allow you to target cards that are in play in front of other players.