So far this is a great show, and i almost never watch anything on TV.
Anyways, this is what i think will end up happening....Spoiler:
I was discussing it with a friend and we came to the conclussion that the polar bears were on the plane crash the happened there 16 years ago. The bears excaped and killed everyone except for the French lady who ran the loop, probably her too later on 
. Then the bears the adapted to the island, thriving due to the large amounts of prey to feed upon. Thus, they grew into enourmus, blood craving, behemoths, for polar bears are the most aggresive bears towards humans in real life, as well as already being the biggest four legged carnivores on earth. In addition, i don't see how the polar bear that was killed could be the only one there, for they said it looked young, especially if the huge monster(s) is a polar bear. If the polar bear was young then how would it get there and be the only one? After all if i penguin can survive on the Galapagose Islands, why can't a polar bear survive near Fiji 
Well those are my thoughts so far about the monster, and I'm hoping that i could possibly get some more feedback on that teory here at TFP.