Mysterious monsters without much explanation?

[Absimiliard]

The game calls Ultima extradimensional. That could mean a lot of things, but the wording and surrounding context lead me to believe it's probably got nothing to do with the thirteen reflections.

Originally Ultima had no mind to speak of. She was simply a thing summoned to the Source by Ajora to destroy Mullonde. It was sealed away by Ramza several centuries later, when it reawoke. The description of her included with her collectible statue says people began to worship her out of fear, which in turn fed her in much the same way prayer might feed a primal. The process awoke in her a sense of self. She had effectively become a primal with an actual living body. The text on Ultima's in-game TT card corroborates the statue's description and adds that she was once the "embodiment of pure ruination."