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It's in the Great Old One Subtype in Bestiary 4 and Bestiary 6.
A Great Old One has mythic power (10/day, surge +1d12) and counts as a 10th-rank mythic creature. A Great Old One can use any of its spell-like abilities as the mythic versions of those spells (if a mythic version of that spell exists), expending mythic power as normal. It can also expend mythic power to use the augmented versions of these spell-like abilities.
It looks like they didn't want to give them the full mythic subtype since that adds other abilities like bonus HP and natural AC in addition to the bonus abilities of the Great Old One subtype.