ORAS remakes Hoenn with Gen 6 mechanics, and the roster plays completely differently from the original Ruby and Sapphire. Fairy type exists now, Mega Evolution adds power spikes, and Primal Reversion turns Groudon and Kyogre into 770 BST monsters. DexNav lets you find hidden abilities in the wild. Same Hoenn region, radically different power levels.
Hypno
Krabby
Kingler
Voltorb
Electrode
Exeggcute
Exeggutor
Cubone
Marowak
Hitmonlee
Hitmonchan
Lickitung
Koffing
Weezing
Rhyhorn
Rhydon
Chansey
Tangela
Kangaskhan
Horsea
Seadra
Goldeen
Seaking
Staryu
Starmie
Mr. Mime
Scyther
Jynx
Electabuzz
Magmar
Pinsir
Tauros
Magikarp
Gyarados
Lapras
Ditto
Eevee
Vaporeon
Jolteon
Flareon
Porygon
Omanyte
Omastar
Kabuto
Kabutops
Aerodactyl
Snorlax
Articuno
721 total, same as X and Y. The Hoenn regional dex is expanded over the Gen 3 originals. Fairy type exists, Mega Evolutions are available, and DexNav lets you find Pokemon with hidden abilities and egg moves in the wild.
Gen 6 mechanics: Fairy type, Mega Evolution, Primal Reversion, and DexNav wild encounters. Primal Groudon and Primal Kyogre hit 770 BST. The Delta Episode postgame adds Rayquaza and Deoxys. And you can fly on Mega Latios/Latias in the Soar feature.
Primal Reversion is like Mega Evolution but exclusive to Groudon and Kyogre. When holding the Red Orb or Blue Orb, they revert to their primal forms automatically upon entering battle. 770 BST each with upgraded abilities: Desolate Land (Groudon) blocks Water moves, Primordial Sea (Kyogre) blocks Fire moves.
Mudkip is still the best. Mega Swampert gets Swift Swim in rain and 150 Attack. One Grass weakness, dual STAB Earthquake plus Waterfall. Blaziken with Speed Boost is the competitive pick, but Swampert is more reliable for the story.
Generation 6, released in 2014 for the Nintendo 3DS. They're remakes of Gen 3's Ruby and Sapphire using the X and Y engine. Same Hoenn region with Fairy type, Mega Evolution, and updated graphics.
DexNav is a tool on the bottom screen that shows which Pokemon you've caught in each area and lets you hunt for rare ones. When a Pokemon is "detected," you can sneak up on it. The detected Pokemon may have hidden abilities, egg moves, or higher-than-normal levels. It's a unique catch mechanic that no other game has replicated.
- Gen IY
Yellow151 Pokemon - Gen IRB

Red & Blue151 Pokemon - Gen IIGS

Gold & Silver251 Pokemon - Gen IIC
Crystal251 Pokemon - Gen IIIRS

Ruby & Sapphire386 Pokemon - Gen IIIFRLG

FireRed & LeafGreen386 Pokemon - Gen IIIE
Emerald386 Pokemon - Gen IVPt
Platinum493 Pokemon - Gen IVHGSS

HeartGold & SoulSilver493 Pokemon - Gen IVDP

Diamond & Pearl493 Pokemon - Gen VBW

Black & White649 Pokemon - Gen VB2W2

Black 2 & White 2649 Pokemon - Gen VIXY

X & Y721 Pokemon - Gen VIORAS

Omega Ruby & Alpha Sapphire721 Pokemon - Gen VIIUSUM

Ultra Sun & Ultra Moon807 Pokemon - Gen VIISM

Sun & Moon802 Pokemon - Gen VIILGPE

Let's Go Pikachu & Eevee153 Pokemon - Gen VIIISwSh

Sword & Shield664 Pokemon - Gen VIIIPLA
Legends: Arceus242 Pokemon - Gen VIIIBDSP

Brilliant Diamond & Shining Pearl493 Pokemon - Gen IXSV

Scarlet & Violet733 Pokemon - Gen IXLZA
Legends: Z-A364 Pokemon - Gen XWW

Winds & Waves51 Pokemon