in season 2 the orientation film showed brief shots of polar bears when explaining the dharma's experiments. and in season 3 kate and sawyer were kept in bear cages on hydra island while jack was kept in an underground chamber with an underwater window like they have in zoos to watch penguins and polar bears swim. i figure they either got out once their keepers were dead or the others just released them to free up the space and they swam to the main island. it seems like there were only a few since we didn't see any of them after year 3.
