So I KO-ed it. Don't look at me it was the underleveled jellyfish and his crit poison sting. Can it be re-encountered or is it gone?
Sorry, but you're out of luck. In Emerald, when you knock out a legendary Pokemon, there's no getting it back. The only way you could manage to get a Regice is restarting your game if you haven't saved after taking out the legendary, trading in one from another game, hacking one in, or restarting your save file entirely.