The bar mace, long iron mace, and studded warclub were blocked from being used on horseback back when they still had crushthrough. They haven't had crushthrough in ages, so (hopefully) there's no reason they can't be used on horseback again. Damage-wise, a bar mace's 35 blunt damage is roughly on par with the morningstar's 38 pierce and the bardiche's 46 cut.
Yaaay for variety!