What's up with Bahamut?

[Enkidoh]

That's consistant with Bahamut's appearence though throughout the FF series where in most FF games he's usually more bipedal than serpentine.

But also remember the Primal Bahamut is a bastardization of what the real Bahamut looked like, so there might have been some liberties with the appearence based on Tiamat's thoughts when he was first summoned (after all, Tiamat was so horrified by the result she allowed herself to be captured by the Allag and remained in confinement on Azys Lla for thousands of years simply as penance for disrespecting Bahamut's memory).

Primals rarely perfectly capture the image of what the summoner has in mind, especially if it's referencing an actual person or entity (another example is the Primal Shiva looking nothing like the historical Shiva Hraesvelgar fell in love with).