Bears have thick fur and skin: They should have both high armor and high health. They should be slow as hell though. Average maneuver makes sense, being quasi-bipedal.
hit points: 180
body armor: 35
difficulty: 6
speed: 30
maneuver: 40
charge: 60
More hit points than plated charger, less armor than barded horses, slowest mount, with okay maneuver, and best charge.
I say difficulty 6 because otherwise you'd only get pansy-ass agi chars riding them, which is not fitting for such a beast. To make up for this, the gold cost would be substantial.
Who would ride bears? I think 1h/2h cav mostly. They wouldn't be good for horsearchers (too slow, vulnerable to cavalry). They would be quite slow for lancing. If I was to ride the bear I created above, I'd get a shield, heavy armor, and a morningstar. Throwing weapons might be useful too.
Although the suggestion seems non-serious, I think it would be a huge boost to the game's popularity. Bear-riding isn't historically accurate, but almost conceivable and this is Calradia after all.