Now, exactly why would the whale be Ghirahim's master? I'm almost 100 percent certain that Link will quite easily destroy that whale as he comes across it, and really a whale wouldn't be befitting of someone of a stature and ego such as Ghirahim.
This beast looks massive enough to possibly bring down Skyloft, though. There may be no way to defeat it 'quite easily' if this is the final boss. I'm only suggesting it as my top pick from what we've thus far seen, though.
Thareous, that's surely Ghirahim's servant. It's in Link's dreams coming to snatch Zelda out of the sky, so clearly those are prophecy dreams. In turn, that means that monster will appear early in the game and will be a normal boss.
Not necessarily. I agree it does appear in in Link's dreams--or visions, I prefer--but the pace of the game could set its sequence in the end. Again, I'm not saying that's concrete. But the Sky Whale could emerge during any point of the game, and therefore is likelier to be Ghirahim's master, IMO.