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.
Mesprit
Azelf
Dialga
Palkia
Heatran
Regigigas
Giratina
Cresselia
Phione
Manaphy
Darkrai
Shaymin
Arceus
Victini
Snivy
Servine
Serperior
Tepig
Pignite
Emboar
Oshawott
Dewott
Samurott
Patrat
Watchog
Lillipup
Herdier
Stoutland
Purrloin
Liepard
Pansage
Simisage
Pansear
Simisear
Panpour
Simipour
Munna
Musharna
Pidove
Tranquill
Unfezant
Blitzle
Zebstrika
Roggenrola
Boldore
Gigalith
Woobat
Swoobat
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