Pantheon's early game is a different kind of strong. He throws spears from range. You can't dodge them. If you stay out of range your not getting any CS or xp in lane.
In contrast, Volibear is 100% melee. He has no gap closer. He has no ranged abilities to farm from a distance. He has nothing to help him escape ganks (I'm assuming you're talking about top lane volibear, because it was top lane pantheon that caused his nerfs).
In my opinion, Top lane volibear is perfectly fine and shouldn't be changed at all. He has lots of weaknesses (most notable the 100% melee thing) to offset his strengths. Jungle volibear on the other hand is perhaps a bit strong. Again though, he's entirely melee. There's really not much he can do if you're paying attention. Even a relatively close ward will give you plenty of time to get away from him because he's only got a small MS boost in his kit (30% and only when towards champions). I'm not sure what makes jungle volibear so strong - maybe it's his passive helping his first clear (reasonable nerf would be that he needs to take champion damage or turret damage to proc his passive?). Maybe it's his fear giving him too much help (revert the jungle item change that let him heal off of feared camps, as before it was only when the monster was in combat [i.e. attacking you]). I really have no idea why he sees such a high win rate in the jungle right now - I just hope if he sees nerfs it's really targeted on jungle volibear as lane volibear feels balanced to me.