Who says Sin looks the same every time it gets reborn? For all we know it could have been something totally different the first few times around.
SpoilerLike a unicorn! After all, the Final Aeons that defeat it are different too, and Sin is built around them.
Speculation aside, the fact that Sin resides mostly in the ocean makes sense for it to be a whale. On the other hand, at some point it grows wings and has some sort of legs so out goes the theory.
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Overall I'm really fond of Sins design. It feels like it could be a real creature. I need to play this game again!