Maybe not a bug per se, but at the least an oddity. Last night, Yardislan was riding his horse (*snickers* Tulip, now a victim to the Broken Forest), able to weild his great sword, but couldn't equip a long sword and a shield.
I could see maybe using a one handed weapon only while riding. And using a one handed weapon and a shield, since even using a shield, your off hand is still free to hold the reins.
Or even a two handed weapon after taking as riding proficiency feat, since you'd be guiding the animal with your legs only *trying to remember the wording from the Player's Handbook/DMG*
This sounds like it might be a bug (or maybe on purpose *shrugs*) with the offhand weapon slot. But are there reports from anybody else who actually takes their horses into combat?
EDIT: Cleaned up the wording since I'm tired and this didn't make much sense before