I sort of like the old sea serpent in the Limsa Lominsa intro. I never thought it was Leviathan (because I assumed Leviathan would look more similar to the newer model), but I did like the effect it gave. I don't think any of us will forget this scene from the benchmark, or from beta. I hope they don't change it.

That said, I LOVE the new Leviathan.

I think the 'old' Leviathan was a generic term for a huge sea serpent. There may be others like it, or at least similar to it. The 'new' Leviathan is the avatar/primal we know and love... completely unique (there are none other like him).