Zodiark's design actually makes a lot of sense, as does the mask the Ancients put on him.

Remember, that during his design and creation, the Final Days had spread everywhere except to Amaurot's region. We know it affects entire regions. Anyway, the people who made him and the people who gave their lives to him were those who had resisted the effects of the Final Days. Resisted an inimical force that affects their very thoughts, beings, and most importantly creation magicks.

So Zodiark was created under a lot of strain and duress. He carried a lot of monstrous things with the gravity of the powers he would possess, that intrinsically came at the cost of so many loved ones.

Imagine having to create a man like being from the remains of your best friend. Of the remains of your wife. Your brothers.

Now imagine you also have to do that while someone holds a loaded and cocked gun to your forehead.

How do you think your creation is going to look?