USUM expands the Alola Pokedex over Sun and Moon. More Pokemon are available earlier, new Ultra Beasts join the roster, and Ultra Warp Ride opens up legendaries from every past generation. The late game is harder too. Ultra Necrozma will sweep your team if you're not ready. Same Trial system, bigger Pokedex.
Yanma
Wooper
Quagsire
Espeon
Umbreon
Murkrow
Slowking
Misdreavus
Unown
Wobbuffet
Girafarig
Pineco
Forretress
Dunsparce
Gligar
Steelix
Snubbull
Granbull
Qwilfish
Scizor
Shuckle
Heracross
Sneasel
Teddiursa
Ursaring
Slugma
Magcargo
Swinub
Piloswine
Corsola
Remoraid
Octillery
Delibird
Mantine
Skarmory
Houndour
Houndoom
Kingdra
Phanpy
Donphan
Porygon2
Stantler
Smeargle
Tyrogue
Hitmontop
Smoochum
Elekid
Magby
807 total, five more than Sun and Moon. The new additions are Ultra Beasts: Poipole, Naganadel, Stakataka, Blacephalon, and Zeraora. The Alola dex is slightly expanded with more encounter options early.
The story diverges around the midpoint. Necrozma absorbs the box legendary to become Ultra Necrozma, one of the hardest boss fights in any Pokemon game. Ultra Warp Ride lets you catch legendaries from every past generation. New Ultra Beasts, more early encounters, and the Mantine Surf minigame are added.
Ultra Necrozma has 167 Sp. Atk, 97 Speed, and gets a stat-boosting aura on top of that. It will sweep most teams. Common strategies: Zoroark's Illusion to bait a Fighting-type move, Toxic stalling, or a Focus Sash Pokemon with a super effective hit. It's level 60 and the biggest difficulty spike in the series.
A minigame where you fly through Ultra Space on Solgaleo or Lunala. Different colored wormholes lead to different Pokemon. Legendary Pokemon from past generations (Mewtwo, Ho-Oh, Groudon, etc.) appear in the furthest wormholes. It's the main way to catch non-Alolan legendaries without transferring from older games.
Still Gen 7, 3DS, 2017. USUM is the enhanced take on Alola. Same Trial system, but the story diverges around the midpoint into Ultra Space content. More Pokemon available earlier, new Ultra Beasts, and a harder endgame stretch.
Same three starters as Sun and Moon. Litten's Incineroar is still the most practical for the story since Intimidate neuters Totem Pokemon. But team building matters more in USUM because Ultra Necrozma demands a specific answer on your squad.
- 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