Ruby & Sapphire Pokedex

Ruby gets Groudon, Mawile, Zangoose, Solrock, and Seedot line. Sapphire gets Kyogre, Sableye, Seviper, Lunatone, and Lotad line. Plus about a dozen other minor differences.

Mudkip and it isn't close. Swampert is Water/Ground with only one weakness (Grass, 4x). It gets Earthquake and Surf for dual STAB and tanks most things. Torchic evolves into the solid Blaziken, and Treecko's Sceptile is fast but frail.

Abilities are passive traits introduced in Gen 3. Every Pokemon species has one or two possible abilities that provide an automatic effect in battle. Examples: Slaking's Truant forces it to skip every other turn despite 670 BST. Swampert's Torrent boosts Water moves when HP is low. Abilities can make or break a Pokemon.

Generation 3, released in 2002 in Japan and 2003 in North America for the Game Boy Advance. Introduced abilities, natures, double battles, and the EV/IV system that still exists today. Set in the Hoenn region.

Groudon (Ruby) vs Kyogre (Sapphire) is the main decision. Kyogre is generally considered stronger for in-game use since Water STAB with Drizzle covers more matchups. But the non-legendary exclusives matter too. Ruby's Zangoose and Sapphire's Seviper are thematic rivals.