Walkthrough
After the triangular meteorite erupts on Birth Island, you will find the Legendary Pokemon known as Deoxys. Deoxys is a Mythical Pokemon that hails from the Hoenn Region, being a pure Psychic Type Pokemon. In Pokemon FireRed, Deoxys will know the moves Taunt, Pursuit, Psychic, and Superpower. In Pokemon LeafGreen, it will know the moves Knock Off, Spikes, Psychic, and Snatch. It will be found at Level 30 regardless of which game your're playing.
Because Deoxys is a pure Psychic Type, it is weak against Ghost Type Pokemon, Dark Type Pokemon, and Bug Type Pokemon. Since it will be found at such a low level, it is possible to capture Deoxys using Poke Balls and Great Balls. That said, however, Ultra Balls still work best since it still is quite strong. Once captured, Deoxys will revert into one of its other forms: its Attack Form in Pokemon FireRed, or its Defense Form in Pokemon LeafGreen.