Emerald Pokedex

Both Team Aqua and Team Magma are antagonists (instead of one per version). Rayquaza is catchable at Sky Pillar before the Elite Four. Wallace is Champion instead of Steven. The Battle Frontier replaces the Battle Tower with 7 facilities. Move tutors teach coverage moves RS didn't have. And some wild Pokemon show up in new locations.

Yes. After the Groudon vs Kyogre showdown at Sootopolis, you go to Sky Pillar to summon Rayquaza. It flies off to stop the fight. Later, after getting the eighth badge, you can return to Sky Pillar and catch it. Level 70, 680 BST. It's one of the strongest Pokemon you can get before the credits.

Mudkip is still the best. Swampert's Water/Ground typing with one weakness carries through the whole game. The Champion is Wallace (Water specialist) in Emerald instead of Steven (Steel), so you need to adjust your endgame team. Sceptile does well against Wallace though.

Gen 3, GBA, 2004. Emerald is Hoenn's third version. The roster and ability system are the same as Ruby and Sapphire, but the story plays out differently (both villains active, Rayquaza resolves the conflict). The Battle Frontier postgame is the main draw for experienced players.

The Battle Frontier is Emerald's postgame challenge facility with 7 different battle formats: Battle Tower, Battle Factory, Battle Pike, Battle Dome, Battle Palace, Battle Arena, and Battle Pyramid. Each has a Frontier Brain boss. It's considered one of the hardest postgame challenges in any Pokemon game.