--Via Mega Evolution
- Venusaur
-Charizard
-Blastoise
-Alakazam
-Gengar
-Kangaskhan
- Pinsir
- Gyarados
-Aerodactyl
- Mewtwo
-Ampharos
-Scizor
-Heracross
-Houndoom
-Tyranitar
-Blaziken
-Gardevoir
-Mawile
-Aggron
- Medicham
- Manetric
- Banette
- Absol
-Latias
-Latios
- Garchomp
- Lucario
- Abomasnow
- Beedrill
-Pidgeot
- Slowbro
- Steelix
- Sceptile
- Swampert
- Sableye
- Sharpedo
- Camerupt
- Altaria
- Glalie
- Salamence
-Metagross
- Rayquaza
- Gallade
-Lopunny
- Audino
- Diance
Via other Held items
-Kyogre (Blue Orb)
- Groudon (Red Orb)
- Dusk and Dawn Nezcroma (Uses Ultranecrozium Z)
Via Weather
- Cherrim (Harsh Sunlight)
- Castform(Any Weather)
Via some condition in Battle
- Darmanitan (Zen Mode Ability, below half HP)
- Meloetta (Uses Relic Song in battle. Only changes to Aria Form. Can't change back until Battle ends or faints.)
- Greninja (Battle Bond Abikity, causes an opponent to faint)
- Aeigislash (Uses Attacking moves for Blade form. King's Shield for Shield form)
- Zygarde (Power Construct Ability, below half HP)
- Wishiwashi- (Reverts to solo form at the end of a turn if below 25% HP)
- Minior ( Core to Meteor Form when sent out to battle or at the end of the turn and above 50% HP. Changes to Core form of below 50% HP)
- Mimikyu (Disguise to Busted Form if hit with an attack)
- Cramorant (Gulping Form- Uses Surf or Dive with at least 50% HP. Gorging Form- Uses Surf or Dive with less than 50% HP. Reverts to normal once hit with a move)
- Eiscue (Ice Face-> Noice Face. Hit with a Physical Move. Ice Face is restored in Hail)
- Morpeko- (Switches between Full Belly and Hangry Mode at the end of every turn)
Others
- Dynamaxing. 3 Turns once you click the Dynamax option. Makes you grow large and increases HP stat. Lets you use Max Moves. Only available in certain areas like Max Raids.
- Gigantimaxing- Like Dynamaxing, but these Pokémon have access to an exclusive move. These are the Pokémon.
Venusaur
Charizard
Blastoise
Butterfree
Pikachu
Meowth
Machamp
Gengar
Kingler
Lapras
Eevee
Snorlax
Garbodor
Melmetal
Rillaboom
Cinderace
Inteleon
Corviknight
Orbeetle
Drednaw
Coalossal
Flapple
Appletun
Sandaconda
Toxricity
Centiskorch
Hatterene
Grimmsnarl
Alcremie
Cooperajah
Duraladon
Urshifu both forms.
Note that Eternatus can Eternamax, but this unavailable except for that 1 battle. I may have missed 1 or 2, if I did, please comment.
Source: https://m.bulbapedia.bulbagarden.net/wiki/List_of_Pokémon_with_form_differences
