Polar bears are well, polar bears. They are big white bears wearing spiked armour. While they are quite slow, they are tough and deal considerable damage. Polar bears are able to damage buildings with their attacks, making them living siege engines.
They can be found in Stronghold Legends, when playing as the Ice faction. They can be trained from the Ice Tower. As far as strategy and usage goes, they can be thought of as weaker and slower, but cheaper, versions of Frost giants.