Whether you want to believe they actually did the things in myth is up to you. A number of them have been shown to possess the powers described in legend, though. Even Mew's role is debatable since it is just a theory that has no reason based on why they say it is the origin.
Me, I follow the legends, but not in the way that most would. Instead of actually being pokemon, some of the more powerful legends are actually embodiments of the aspects they represent. In other words, the pokemon we see is just a form they chose to manifest as in the world. Mew is the first of the truly flesh and blood creatures known as pokemon, created by Xerneas. This was the point at which their existence became limited because of the presence of Yveltal. Starting with Mew, no pokemon could be immortal.
What came before, then? There are some things I haven't fully ironed out, but obviously, Xerneas, Yveltal, and Zygarde. With them, I include Groudon, Kyogre, and Rayquaza, whose forces formed the pokeworld itself. (screw you, OR/AS, I'm still going along with this)
Then there's Reshiram, Zekrom, and Kyurem, the components of the Original Dragon, who for me takes on the role of nature in it's entirety. This includes space and other worlds as well. (so, the universe in essence) Technology is the manipulation of nature's forces, so when humanity created it, it caused the original dragon to split to the force that keeps nature as it is (Reshiram) and the one that changes it (Zekrom). I know they're supposed to be truth and ideals, but in a way, this is still truth and ideals. Kyurem is the empty husk, the void, a black hole. Remember that cold is the absence of heat.
(continued)
