Here's how you can get Articuno in Pokemon Ruby:
1. Complete the main story: You need to have defeated the Elite Four and become the champion.
2. Head to the Seafloor Cavern: This cave is located at the bottom of the ocean in the far southwest corner of the map.
3. Find the Legendary Bird Trio: Once you enter the cave, you will encounter Articuno, Zapdos, and Moltres. You can choose which one to battle and potentially catch.
Tips for catching Articuno:
* Use a Master Ball: This is the most reliable way to catch a legendary Pokemon.
* Lower its HP: Use moves like "False Swipe" to bring Articuno's HP down to a small amount.
* Throw Ultra Balls: These have a good chance of capturing a legendary Pokemon.
* Use berries: Berries like "Razz Berries" can increase your chances of catching Articuno.
* Be patient: Catching a legendary Pokemon can take several attempts.
Note: You can only catch one Legendary Bird in a single save file.