* Orcas: Orcas have been known to kill polar bears in the water, though this is rare.
* Walruses: Large bull walruses might defend themselves against a polar bear, potentially causing serious injury or even death.
* Brown bears: In areas where their ranges overlap, brown bears and polar bears could potentially fight, but this is also uncommon.
* Wolves: Wolves might scavenge a dead polar bear, but they wouldn't be able to kill a healthy adult.
It's important to remember that these are all rare occurrences. Polar bears are incredibly powerful predators, and they are generally at the top of the food chain in their Arctic habitat.