Originally these forms were available through putting down a torch,throwing out your desired Halloweenafied pokemon and waiting till exactly Midnight during a full moon and using a Jack-o-Lantern on the pokemon you desired to be changed. It would then get the Special "Skin" if it had one available. Sadly with Pixelmon generations the code strings attached to this special event arent currently working. However Form Change Scrolls Do for some if not all of these pokemon below.

You can currently View what the models look like using a chisel in-game


Pokemon with special halloween Skins
Venusaur
Charizard
Blastoise
Meganium
Typhlosion
Feraligatr
Sceptile
Blaziken
Swampert
Torterra
Infernape
Empoleon
Serperior
Emboar
Samurott
Chesnaught
Delphox
Greninja
Decidueye
Incinderoar
Primarina
Ralts
Kirlia
Gardevoir
Espeon
Glaceon
Leafeon
Flareon
Lugia (shadow)
Ho-Oh
Mewtwo
Haunter
Gengar
Shuckle
Weavile
MrMime
Onix
Keckleon
Togedemaru
Sudowoodo
Mimikyu
Butterfree
Tangrowth
Meowth (Ninja) (Has to be edited to form 6 )
Persian (Ninja) ( Has to be edited to form 6 )
Pikachu
Ditto
Weavile ( Has to be editted to form 8 )
Alolan Vulpix ( Not currently available as far as i could find the strings weren't attached to the model)
Easter Lopunny
Easter Buneary
Zelda Aegislash
Zelda Masquerain
Kurama Ninetales
Ying-Yang Absol
Shadow Arcanine
Golden Bidoof
Primal Dialga
Shadow Mew
Rainbow Rapidash
Shadow Vaporeon
Purple Tyranitar
Pastel Gothita
Pastel Gothitelle
Pastel Gothorita
Purple Scizor
Special Gallade
BaldEagle Braviary
Joker Zoroark
Special Staraptor
Picklemyuku
Husky Rockruff

Extra forms that arent Halloween Based -
Magearna - Original
Shadow and Shiny Shadow Lugia
Spectral Haunter
These have been shown in the Sneak Peak and ill be updating this post when i see where the files are actually Localized for each of these extra models and put them in the appropriate areas.

Note: None of the forms above with available Mega Evolutions have the Mega Evolution Altered. It will show up as the base version.