It sounds like you might be thinking of one of the Legendary Titans:
* Regirock: You can catch Regirock in the Ancient Tomb after you have found the Regirock Key. This involves solving a puzzle by pushing three stones in a specific order.
* Regice: You can catch Regice in the Ancient Tomb after you have found the Regice Key. This also involves a puzzle where you need to push three stones in a specific order.
* Registeel: You can catch Registeel in the Ancient Tomb after you have found the Registeel Key. This involves solving a puzzle by pushing three stones in a specific order.
These are the only Legendary Titans available in Pokemon Emerald.
If you have any other questions about these Pokemon or the game, feel free to ask!