The Japanese fishermen didn't want "it" (whatever it is) stinking up the fish they had caught, so they threw it back. It has stirred up a lot of controversy over the years. Many believe it was a dead plesiosaur (see skeleton below), a believed-to-be extinct marine dinosaur. The nay sayers declare it to be a basking shark.
Whatever it was, it is an interesting topic to discuss in science class.