Another random thought we have (quality debates please left here) mythic spells mythic diseases, mythic monsters and we know mythic characters are empowered by special energies above and beyond normal heroes (divine heritage, ancient monuments, etc). So do you think all "Mythic" things contain some "mythic energy" or can some simply be "mythic" in the sense of myths and legends. For example in call of the wild (novel) the dog becomes a myth as a great ghost dog told about by the tribes in the area hunting and avenging his masters death. So do you think he could be considered a mythic being even though he's a perfectly ordinary dog or would he have to be empowered somehow to count?
When I first started running Mythic Adventures, I made every mythic creature unique with its own legends and backstory. This worked fine as long as I had solo mythic creatures, but once I started using multiple mythic creatures, it became too cumbersome to give them all stories. They just became "rare" versions which were much more powerful than their non-mythic counterparts.
It's the difference between mechanical game "mythic" and literary "mythic". It's metagame. Most things don't have game mythic either because they existed before those rules were added or because those rules were not well received and so not added to a lot of later creatures.
To add on to this, the Elder Wyrm (Bestiary 6) has an ability called Myth-Drinker that gets an increase of damage and can drain mythic points against Mythic targets, but it also states:
At the GM’s discretion, this ability could apply to mighty albeit non-mythic divine creations, such as behemoths, demodands, titans, and outsiders whose CR is 20 or higher; in this case the elder wyrm benefits as though the target had lost two uses of mythic power
Monsters like the Spawn of Rovagug are not outside the purview of being considered Mythical, as stated earlier that they were made before these rules existed.