
Originally Posted by
Sir Bahamut
There may have been a "real" Griever that existed in stories that Squall had known about, but the only suggestion offered in-game is that it is a creature that Squall conceived of in his own mind, as Scan tells us that in Squall's mind, Griever is the strongest GF:
"Griever In Squall's mind, the strongest GF. Through Ultimecia's power, continues fighting without vanishing."
Further, Ultimecia's Witch Embodiment powers granted her the ability to reach into other's minds and pull things out (as she often displays when completely removing a character's stock of a certain type of Spell). In the case of Griever, she simply manifested the thoughts she pulled from Squall's mind regarding what he believed to be the most powerful being in existence. This would be a great strategical move on Ultimecia's part. This is made even more plausible when examining what the japanese version has Ultimecia say:
Ultimecia: Your feelings, I shall summon the most powerful of things [from them]! The more strongly you feel, that will be what shall torment you. Fufu.