Belugas eat fish. Fish have bones. Or maybe this one ground its teeth in its sleep?
It was the hairy stuff that was a bit of strangeness on this one. However, Cryptomundo said that these whales have some subcutaneous fibrous structure below their outer layer of skin, so that seems in order with the level of decomposition. Not a plesiosaur.
I'm tempted with the whale theory too , but think that there are still unknown creatures in the ocean which may one day be discovered and explain some of the 'sea monster' sightings. I've seen basking sharks off the west coast of Scotland (an area notorious for sea monster reports) and have to admit that the first time you see one , you think you're seeing something unknown (like a sea monster). The mind can play tricks , as can misidentification , but who knows what is out there awaiting discovery ?
good lord! precisely my first thought was DARK CRYSTAL! if you read the comments further down the link, a lot of people say the same thing- they are called Skeksis, it looks like a combo of the Skeksis and their "good" counterparts, the Mystics. of course that's not what the creature actually is, but if you haven't seen the movie, i highly recommend it-- very sinister for a children's film.