It should be noted that the trio of bosses fought before Ultima (Mustadio, Agrias, and The Thunder God) are the result of heroic individuals using Ultima's auracite. Much like the zodiac stones they were adapted from, they're not inherently evil things, but rather their effects are dependent on the user's intentions and possibly willpower. And likewise, Ultima's auracite went on to be the basis of "trigger weapons"—devices designed to streamline the process of summoning primals—made by the Allagan Empire. That provides a definite link between Ultima's auracite and primal summoning, and could suggest that Belias and friends are also primals in a fashion.
Or they could be voidsent. Or voidsent primals.