Meditite is a Psychic and Fighting type Pokemon that is not available for capture in Pokemon Emerald. It is however in Pokemon Ruby and Sapphire, so it may easily be traded to Emerald from another game.
Google: How to clone pokemon in pokemon emerald. Be warned, this may ruin the fun for you, and it does work. Be very very careful. Think of it as a last resort, once you have had all the fun you can in emerald, then use this cheat.