The fact that Cagnazzo is a giant turtle with humanoid appendages and a humanoid head is exactly what makes him look so terrifying. He is that which should not be.

Also, I've never thought of the bosses whom you happen to fight to get to the Crystals in the first world as FFV's equivalent to the Four Emperors. If anything, those would be Exdeath's personal cadre of demons, including Melusine, Stalker, the triplets, and the unnamed red devil who gets taken out by Leviathan. There's much more than four of them, but their role is the same.