Thanks to brauchen for the inspiration!
Maduin (Madeen) is Griever.
For Madiun to be Griever, we need more than the world. Maduin dies in Final Fantasy VI and fades away once magic disappears. For Maduin to be Griever, Final Fantasy VIII would have to come before Final Fantasy VI in the timeline. But in Final Fantasy VIII, Griever is:
not only a symbol of the virtues Squall values, but is also Squall’s interpretation of the ultimate Guardian Force
This means that Griever didn’t come into existence until Ultimecia brings him forth. Not only does this directly contradict the lore of Espers in Final Fantasy VI:
A thousand years ago three gods, known as the Warring Triad, descended upon the world and warred for dominance. During the War of the Magi the gods transformed humans and animals into creatures called espers, giving them magical powers. The gods realized the war was destroying the world and petrified themselves, their final wish is that the espers prevent their power from being abused. The espers took the gods’ statues and fashioned a new dimension where they could live peacefully away from humans, and to hide away the gods.
but it also prevents Griever from becoming Maduin in the future, as Squall defeats him.
However, maybe Maduin was the inspiration for Griever? They both look similar. Maduin’s sprite looks similar to the stance of the Gigas. Perhaps Yoshitaka Amano put a bestial spin on the design:
Maduin is a humanoid esper with long sea-greenish-gray hair on his field sprite, though on his summon sprite it’s grayish-violet. Both field and summon sprites depict him with his hair tied up in a ponytail, and tufts of fur the same color as his hair on his shoulders. He has a muscular figure, with he summon sprite depicts him with a grayish-tangelo skin tone and his field sprite with light gamboge skin. His summon sprite shows him with clawed fingers and digigrade legs with long talons on his toes. Maduin wears a dark vermilion loincloth with an amber sash in his summon sprite, and wears a pendant he gives to Madeline when they meet. On his field sprite, his eyes are moderate indigo. His summon sprite resembles that of the Gigas from Final Fantasy V, and maybe a heavily modified Gigas sprite.
Amano and Nomura reinterpret each other’s style. Nomura could well have done the same here.