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What you can do tho, is either catch a new one or breed the pokemon and hatch an egg.
Also If you want to prevent a Pokemon from evolving you can give them an everstone.
Most of the time the pokemon hatched from the egg will be its first stage, and it will evolve normally unless you give it an everstone.
Which pokemon are you trying to get?
If you look on your Pokedex, it will tell you where you can catch Ralts and then you can evolve them into a Kirlia and give it an everstone if you don't want it to evolve into a Gardevoir.
Hope this helps.