Here’s something that came to my mind: What if Eternatus is the codename for the mythical and the third legendary is still yet to be mentioned? When I first saw the registered trademark for Marshadow, I thought that would be the name for the third legendary when it was actually Necrozma. I may be overthinking it, but what if the third legendary is just another wolf that is an evil version of the two mascot legends? Like how there’s Dialga/Palkia and Giratina is considered the vengeful dragon of the main heroes. For all we know, Eternatus could just be a made up name.