>>42619478Mythicals are originally done as distribution only, meaning you have to take part in an event or other special method to get them. After their initial release, some have been put into games to be available normally.
Legendaries are just rare Pokemon that are generally limited to one being obtained per game, can't breed, and have high stats. They're typically tied to in game events, though there are exceptions, like the Legendary Birds in RGBY just sort of being there.
Also, we're typically told by TPC when a Pokemon is revealed if it's Legendary or Mythical, as these are big marketing points. I'm not even sure Eternatus has been officially revealed.
