If you're talking about The Pokemon Moon RPG, you can get PrimalRayquaza, PrimalKyogre, and PrimalGroudon in World's End Story from the Battle Area.
to evolve them you have to click on the link that says evolve Pokemon on the right hand side. HOPE THIS HELPS!!! Here's a better explanation. During a battle after, you've used an attack, scroll down...
Not a real (legal) game.
The computer
you get a mushroom and a frog and pee on them and then you will get everything