To decide whether or not the Sea Serpent seen in the Final Fantasy 14 trailer is Leviathan or not, we just need to take a look back at the previous series and spot any connections. Leviathan has appeared in more or less every Final Fantasy title in some form, noteably in some as a summonable avatar. In Final Fantasy XI, Square Enix's previous online game, they featured Leviathan as one of the eight avatars that the Summoner job could summon. Whether or not Leviathan will appear in Fourteen needs confirmation, although it seems pretty obvious.

First of all, let's take a look at Leviathan from Final Fantasy XI. It's a Sea Serpent that has webbed wing-like structures protruding from what's seemingly its shoulders and has spines, or spikes, if you will, running down its back. Its tail is coiled like a snake's and it has the "moustache" like decoration coming from it's chin. The "horns" or "ears" (it's hard to tell which :p) coming from its head are very apparent as well.

Summoners will remember the animation for Spinning Dive, one of Leviathan's abilities. It swims up in the air and coils around, diving down on its prey. In the trailer, the Sea Serpent is seemingly doing this as well, although that's unconfirmed as the trailer cuts off early.

So anyway, let's take a look at Final Fantasy XI's Leviathan:

Now, if we take a look at the Final Fantasy 14 Sea Serpent, the similarities are pretty obvious. The Sea Serpent's head has "moustache" decoration, like Eleven's Leviathan, and it's body is snake-like and enormous in size. Most noticeably, though, it has the webbed wing structures coming from exactly the same spot as Eleven's Leviathan. It also has the ears on its head.

As circled in the image above, the similarities between the Sea Serpent in the trailer and the previous Leviathans are obvious. For anybody not to assume this is Leviathan, they'd have to have inside information. We could be wrong, of course - it could be a completely different creature. But regardless of the name they choose to give this Sea Serpent, it'll be pretty apparent where their inspiration came from.

Whether or not this is indeed Leviathan remains to be seen, but by comparing Leviathan to the Leviathan of the previous series, it'd be a safe bet to assume that the Sea Serpent is indeed Leviathan.