* Bagon doesn't learn Rock Slide: Bagon's movepool in Pokémon Ruby does not include Rock Slide. It learns other Rock-type moves like Rock Tomb and Ancient Power, but not Rock Slide.
* Rock Slide is a later-generation move: Rock Slide was introduced in Generation IV (Pokémon Diamond & Pearl). Pokémon Ruby is a Generation III game.
How to get Bagon in Pokémon Ruby:
1. Find a Bagon: Bagon can be found in the Granite Cave (after defeating the 7th Gym Leader).
2. Train it: Bagon evolves into Shelgon at level 30 and then Salamence at level 50.
To get Rock Slide on a Salamence in later games:
* You'll need to trade your Salamence to a game where Rock Slide is available.
* Then, use a TM or a Move Tutor to teach it Rock Slide.