In my opinion, the leviathans can be seen as two different species. Dragons or Whales. I know a leviathan is a giant sea creature, however, in the game all you see is their bones so it could be a guess made by the people in the game. For the dragon theory, there are three main things to point out. firstly, these dragons relate to the dragons in skyward sword. Secondly, the locations of the dragons include an ice formation in a cave, the foot of a volcano and the desert. The dragon in the cave could be Faron as she is frozen in a cave that could possibly be filled with water prior to an ice age. The dragon at the foot of the volcano can easily be Eldin as the excuse is that there was a volcanic eruption. This doesn't mean that he was hit by the eruption but rather he was shot out of the volcano. And finally, Lanayru, do I have to explain. HOWEVER, there is a fourth dragon in skyward sword, Levias. With some intense searching, I found an area to prove it all. This dragon is all but forgotten because it is located in dragon bone mire. For the whale theory, I'm not talking about 'whales', but however, the wind fish and the ocean king. There is a problem with this however, there are three leviathan bones and only 2 'whales'. The final 'whale' is called, well a whale, known as the sky whale. This is killed in a remote location but thanks to a drought could be the leviathan found in Gerudo desert. These other two leviathans could be at either location as it is unknown where they go when link awakens. If your wondering, the dragon bone mire for the whale theory would still be Levias as he can be seen as a whale and the three dragons, Faron, Eldin and Lanayru could die in an odd way, like disappearing and reincarnating as a new dragon, Farosh, Dinraal and Nydra.