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
Meowth (Ninja) (Has to be edited to form 6 )
Persian (Ninja) ( Has to be edited to form 6 )
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)
Extra forms that arent Halloween Based -
Magearna - Original
Shadow and Shiny Shadow Lugia
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.