geogator: to be honest, i didn't use a full-developed rule, i would have done it in 3.5, but monster creation is much more free, it seems to me, in 4e than before. Basically, i took a hobgoblin soldier and put it on a riding horse. Now, you will notice that riding horses don't offer special advantages as mounts, and that i didn't make the horse attack. What i did is basically give the horse a benefit similar to "ride-by attack" and rule that a skilled rider can help his mount with a move action in various ways. I know the rules don't allow this, but I didn't feel it is too unbalancing. Obviously, the guy is worth more than tthe xp of the two monsters in the MM. Maybe my answer disappointed you, but if someone's interested in mounts i can think about some houserule...
Is: I completely forgot about that. Meriadeth's attack'll hit. I'llo go fix it.