I mean if you knock it out or run away, you could make it respawn somehow.
Starting in Generation III, if the player runs from a Legendary Pokémon, the Pokémon reappears when the player leaves and re-enters its location, Starting in Pokémon Platinum, if the player defeats or runs from an in-game event Pokémon, then it reappears next time the player enters the Hall of Fame.
source This means you have one chance to catch legendaries in gen 2, and you can catch legendaries in gen 3 and DP if you ran from them before.