This page serves to document the additional forme changes and to document how the form system works.

Firstly, the form system in HGSS is very all over the place. Each individual system (icons, learnsets, personal data, etc.) has some sort of appending to the original narc that occurs that contains the form data. Most functions then have a massive if statement table to determine what the actual index is. Icons specifically have 50 extra slots after the original 493.

As a result of this, expanding this system starts at 544. Expanded Pokémon indices go from 544 (Victini) to 955 (Enamorus). New form data, in all narc files, then resumes at 956. Forms for Gens 3 and 4 still use their sprite and icon data from the vanilla game for the most part (as that is still handled by the if statements), but new forms are handled by the table PokeFormDataTbl in src/pokemon.c. This is an array of FormData structures and specifies the base species, the form number, whether or not the form should be reverted after battle, and the new species that the form data will come from.

The FormData structure:

struct FormData
{
    u16 species;
    
    u16 form_no:15;
    u16 need_rev:1;
    
    u16 file;
};

This structure is then iterated through to find which file to read from for Pokémon with forms.

Adding new forms is then exactly just like adding new species, just adding the new data to all the folders necessary. We use SPECIES_* constants just about everywhere we can--the new form data is no exception, with each form data getting a constant, i.e. SPECIES_MELOETTA_PIROUETTE or SPECIES_KYUREM_BLACK.

Venusaur
Charizard X
Charizard Y
Blastoise
Beedrill
Pidgeot
Alakazam
Slowbro
Gengar
Kangaskhan
Pinsir
Gyarados
Aerodactyl
Mewtwo X
Mewtwo Y
Ampharos
Steelix
Scizor
Heracross
Houndoom
Tyranitar
Sceptile
Blaziken
Swampert
Gardevoir
Sableye
Mawile
Aggron
Medicham
Manectric
Sharpedo
Camerupt
Altaria
Banette
Absol
Glalie
Salamence
Metagross
Latias
Latios
Rayquaza (can mega evolve when it knows Dragon Ascent)
Lopunny
Garchomp
Lucario
Abomasnow
Gallade
Audino
Diancie

Kyogre
Groudon

Rattata
Raticate
Raichu
Sandshrew
Sandslash
Vulpix
Ninetales
Diglett
Dugtrio
Meowth
Persian
Geodude
Graveler
Golem
Grimer
Muk
Exeggutor
Marowak

Meowth
Ponyta
Rapidash
Slowpoke
Slowbro
Farfetch'd
Weezing
Mr. Mime
Articuno
Zapdos
Moltres
Slowking
Corsola
Zigzagoon
Linoone
Darumaka
Darmanitan (both Zen Modes included)
Yamask
Stunfisk

    Cosplay
    Rock Star
    Belle
    Pop Star
    Ph.D
    Libre
    Original Cap
    Hoenn Cap
    Sinnoh Cap
    Unova Cap
    Kalos Cap
    Alola Cap
    Partner Cap
    World Cap

    Red Stripe is form 0
    Blue Stripe
    White Stripe

    Spring is form 0
    Summer
    Autumn
    Winter

    Spring is form 0
    Summer
    Autumn
    Winter

    White
    Black

    Douse
    Shock
    Burn
    Chill

    Meadow is form 0
    Polar
    Tundra
    Continental
    Garden
    Elegant
    Meadow
    Modern
    Marine
    Archipelago
    High Plains
    Sandstorm
    River
    Monsoon
    Savanna
    Sun
    Ocean
    Jungle
    Fancy
    Poké Ball

    Red Flower is form 0
    Yellow Flower
    Orange Flower
    Blue Flower
    White Flower

    Red Flower is form 0
    Yellow Flower
    Orange Flower
    Blue Flower
    White Flower
    Eternal Flower

    Red Flower is form 0
    Yellow Flower
    Orange Flower
    Blue Flower
    White Flower

    Natural is form 0
    Heart
    Star
    Diamond
    Debutante
    Matron
    Dandy
    La Reine
    Kabuki
    Pharaoh

    Medium is form 0
    Small
    Large
    Super

    Medium is form 0
    Small
    Large
    Super

    Baile is form 0
    Pom Pom
    Pa'u
    Sensu

    Day is form 0
    Midnight
    Dusk

    Strawberry Sweet is form 0
    Berry Sweet
    Love Sweet
    Star Sweet
    Clover Sweet
    Flower Sweet
    Ribbon Sweet

    Ice Rider
    Shadow Rider

    Normal is form 0
    Sunny
    Rainy
    Snowy

    50% is form 0
    10%
    10% Power Construct
    50% Power Construct
    10% Complete
    50% Complete

    Red is form 0
    Orange
    Yellow
    Green
    Blue
    Indigo
    Violet
    Core Red
    Core Orange
    Core Yellow
    Core Green
    Core Blue
    Core Indigo
    Core Violet

    Base is form 0
    Dusk Mane
    Dawn Wings
    Ultra Dusk Mane
    Ultra Dawn Wings

    Base is form 0
    Gulping
    Gorging

Growlithe
Arcanine
Voltorb
Electrode
Typhlosion
Qwilfish
Sneasel
Samurott
Lilligant
Zorua
Zoroark
Braviary
Sliggoo
Goodra
Avalugg
Decidueye

Unfezant
Frillish
Jellicent
Pyroar
Meowstic
Indeedee
Basculegion
Oinkologne

    Base is Family of 4
    Family of 3

    Green is form 0
    Blue
    Yellow
    White

    Zero is form 0
    Hero

    Curly is form 0
    Droopy
    Stretchy

    Four Segment is form 0
    Three Segment

    Base is form 0
    Roaming

    Base is form 0
    Paldean

    Base is form 0
    Combat
    Blaze
    Aqua

    Base is form 0
    Segin
    Schedar
    Navi
    Ruchbah
    Caph

    Base is form 0
    Antique

    Base is form 0
    Wellspring Mask
    Hearthflame Mask
    Cornerstone Mask

    Base is form 0
    Terastal
    Stellar

    Base is form 0
    Bloodmoon

Along with new forms, various new methods to change specific Pokémon forms have also been implemented. These are discussed below.

While Gen 4 was the generation that introduced gender differences, the things that could differ between genders was very limited. As a result, significant gender differences (that at least change icons between genders) require separate forms.

Unfezant, Frillish, Jellicent, Pyroar, Meowstic, Indeedee, and Basculegion are currently the only ones this applies to. These wild Pokémon--when generated--will swap between the forms at a 50/50 rate except for Pyroar, which is female 7/8 of the time in accordance with the gender split of future generations. This is governed in src/pokemon.c in UpdatePassiveForms.

Tranquill, Litleo, Espurr, and White Stripe Basculin all have their evolutions tweaked to force females to evolve into the female form. Specifically, when a female evolves, it will set its form to the female form. This is done by an evolution table overhaul that allows us to specify forms as such:

evodata SPECIES_LITLEO
    evolutionwithform EVO_LEVEL_MALE, 35, SPECIES_PYROAR, 0
    evolutionwithform EVO_LEVEL_FEMALE, 35, SPECIES_PYROAR, 1
    evolution EVO_NONE, 0, SPECIES_NONE
    evolution EVO_NONE, 0, SPECIES_NONE
    evolution EVO_NONE, 0, SPECIES_NONE
    evolution EVO_NONE, 0, SPECIES_NONE
    evolution EVO_NONE, 0, SPECIES_NONE
    evolution EVO_NONE, 0, SPECIES_NONE
    evolution EVO_NONE, 0, SPECIES_NONE
terminateevodata

Specifying a form of 0 in the evolution doesn't actually change forms upon evolution. This can be solved by specifying an invalid form for the Pokémon in the form field, such as is done for White-Stripe Basculin, with form 0, to evolve into Basculegion:

// evolution methods/parameters are placeholder until those have been implemented
evodata SPECIES_BASCULIN_WHITE_STRIPED
    evolutionwithform EVO_LEVEL_MALE, 0, SPECIES_BASCULEGION, 2 // set to invalid form to force loading male form assets
    evolutionwithform EVO_LEVEL_FEMALE, 0, SPECIES_BASCULEGION, 1
    evolution EVO_NONE, 0, SPECIES_NONE
    evolution EVO_NONE, 0, SPECIES_NONE
    evolution EVO_NONE, 0, SPECIES_NONE
    evolution EVO_NONE, 0, SPECIES_NONE
    evolution EVO_NONE, 0, SPECIES_NONE
    evolution EVO_NONE, 0, SPECIES_NONE
    evolution EVO_NONE, 0, SPECIES_NONE
terminateevodata

Every mega evolution is actually just a form of the base Pokémon. When holding the specific mega stone for the species, the Pokémon will mega evolve.

Deerling and Sawsbuck in the PC do not change form. They only change form in the party.

They update at the same time as the checks for Shaymin that would revert to Land forme would update--time needs to move forward for it to actually change, moving backwards will not properly update the party.

Wild Deerling and Sawsbuck are generated according to the current season, which is just Spring - Summer - Fall - Winter, in that order, starting from January:

Darmanitan with their Hidden Ability bit set get the ability Zen Mode. This allows for Darmanitan to swap between Zen Mode and Normal Mode. Zen Mode activates in-battle when Darmanitan's HP falls below half, turning it to Zen Mode Darmanitan. Galarian Darmanitan will become Galarian Zen Mode Darmanitan as well.

Kyurem that have the DNA Splicers used on them set a chain of events off:

• First, it checks for the forme of the Kyurem.
• If the forme is 0, then it looks for a Reshiram or a Zekrom in the party starting from the front going to the back.
  • If it finds one, then it stores the Reshiram or Zekrom in the save as it is. A flag (separate from the scripting flags) also in the save is set that tells us that a Reshiram/Zekrom is stored.
  • The Reshiram or the Zekrom is deleted from the party.
  • The Kyurem then transforms into the respective forme (White or Black depending on if Reshiram or Zekrom was chosen).
  • Finally, Scary Face and Glaciate are replaced with Fusion Flare/Bolt and Ice Burn/Freeze Shock, respectively (if they are present).
• If the forme is not 0 and a Reshiram/Zekrom is stored in the save properly, then it looks for a free party spot starting from the front going to the back.
  • If there is a free spot, then the Reshiram or Zekrom is added to the first available free spot.
  • The Reshiram or the Zekrom is deleted from the save and the flag is cleared.
  • The Kyurem then reverts to forme 0.
  • Fusion Flare/Bolt is replaced with Scary Face, and Ice Burn/Freeze Shock is replaced with Glaciate.

While there is currently no way to transform Keldeo Ordinary to Resolute, Keldeo Resolute, upon forgetting Sacred Sword, will transform back into Keldeo Ordinary.

A Meloetta using Relic Song changes into its Pirouette Forme. When switching out or fainting, the Meloetta changes back to its normal forme.

A Genesect given any one of its Drive items will have that drive inserted on its head.

Tornadus, Thundurus, Landorus, and Enamorus--when a Reveal Mirror is used on them--will transform to and from their Therian formes and their Incarnate formes.

A Greninja with form id 1 will have the Battle Bond ability. In battle, said Greninja will transform into Ash Greninja when a move it uses directly causes another Pokémon to faint. This reverts upon fainting or the battle's ending. However, a Greninja that has transformed can still retransform upon revival in battles.

Wild Vivillon will default to Meadow form until the player sets their location in the GTS. Upon doing this, the Vivillon form will change to emulate how it is described in Bulbapedia. The full list of countries that give a specific form are:

Aegislash with Stance Change will transform into Blade Form when attacking in battle, and Shield Form when using King's Shield in battle. It also reverts when switching and on battle end.

Pumpkaboo and Gourgeist automatically randomly choose between their forms in wild battles.

In battle, Xerneas turns into Active Forme. It reverts at the end of the battle.

A Rockruff that evolves at Night will evolve into Lycanroc's Midnight form. When evolving during the day, the Daytime form will be the target.

A Rockruff with form 1 is an Own Tempo Rockruff. This can only evolve into Lycanroc's Dusk Form when it levels up during Dusk, which is the hour of 5PM.

A Toxel with any of the Low Key natures will evolve into a Low Key Toxtricity. The other natures will evolve into an Amped Toxtricity.

The Low Key natures are as follows: Adamant, Brave, Docile, Hardy, Hasty, Impish, Jolly, Lax, Naive, Naughty, Quirky, Rash, and Sassy.

Sinistea, Polteageist, Poltchageist, and Sinistcha all have a 5% chance of generating as their authentic form. There is no visible difference.

When damaged in battle by a physical move, the Ice Face ability activates and the Eiscue is transformed into its Noice Face form. This prevents the damage from being applied.

If Hail then starts after the Ice Face has been broken through, the Ice Face will regenerate.

A wild Dunsparce or Dudunsparce has a 1% chance of being a three-segment form Dudunsparce instead of the normal four-segment.

A wild Tandemaus or Maushold has a 1% chance of being a family of three Maushold instead of the normal family of four.

A wild Tatsugiri will randomly be any of its Curly, Droopy, or Stretchy forms.