Polar Bears seem to attack Foxes, but the behavior is very inconsistent
Adult Polar Bears seem to only attack them if they're right next to them, and it usually completely fails because they just play a bugged version of the attack animation that does no damage. Even when it works, they only hit them once and then the fox runs away.
The Polar Bears can't catch them, because they walk very slowly 1 step at a time when chasing foxes.
Baby Polar bears can even chase the Foxes to attack them (walk normally), but then they stop because they have to be near their parents at all times