Pokémon Ranch is a game for the Wii where you can transfer Pokémon from your main series games to a virtual ranch. The goal is to raise your Pokémon and earn points to unlock new areas and features.
However, legendary Pokémon cannot be transferred to the Ranch. The game is designed for raising and collecting non-legendary Pokémon.
You can catch legendary Pokémon in the main series games (like Pokémon Diamond, Pearl, Platinum, etc.) and use them in those games. But they cannot be transferred to Pokémon Ranch.