Nice meow. I really liked that show when it was out (assuming it's not any more).
I honestly think horse xbows are a legit class, but I think it should be slightly nerfed. The only thing I can think of is to make reload times take 30% longer on horse (polearms are 30% slower to thrust on horse than on foot).
Or (this is my suggestion and not nerf anything else) make it so your turn radius is slower (basically less agile) while you are reloading. If you have both hands reloading, and are steering the horse with your body and stirrups, you wouldn't be able to make the turn on a dime moves that arabian cav can make (at least IMO).