Hello everyone! Recently I was breeding a BP Blue Flabebe, but kept getting Red. I asked an admin to change color, and I was informed that the colors of forms ~MIGHT~ be applicable to the form scrolls. After a quick test, I learned they were and started to experiment with what exactly form scrolls can do. This is my findings.
One thing I want to clarify before jumping in, some Pokemon have "Skins" while others have "Forms". The best example I can think of is Shadow Lugia and Shadow Ho-Oh. Shadow Lugia is a form, as it was seen in a Pokemon game prior to Pixelmon, so you can use a form scroll in this case, but Shadow ho-Oh is a Skin added by the Pixelmon team, thus a form scroll WILL NOT work to get this Pokemon. Form Scrolls also do not work on Pokemon with gender differences, such as Butterfree. For those, use breeder scrolls.
Legendary Pokemon Forms:
Lugia: You can use a form scroll to get Shadow Lugia, or to revert it back to a Normal Lugia.
Meloetta: You can use a form scroll to get the Pirouette Meloetta, or to revert it back to Aria Meloetta.
As of publishing this guide, Relic Song will not work to get this transformation in Pixelmon.
Keldeo: You can use a form scroll to get Resolute Keldeo, or to revert it back to Ordinary Keldeo.
As of publishing this guide, Sacred Sword will not work to get this transformation in Pixelmon.
Rotom: You can use a form scroll to get the Mow, Wash, Fan, Fridge, and Oven forms, or revert it back to Ordinary Rotom.
As of Publishing this guide, This is the only way to change Rotom forms on Harmony
Magearna: You can use a form scroll to get the Special Magearna, or to revert it back to Ordinary Magearna
Non Legendary Pokemon Forms:
Aegislash: You can use a form scroll to swap between the Sword and Shield forms of Aegislash
As of publishing this guide, Aegislash does not transform as he is supposed to in battle, however he does spawn in both forms, so you can still obtain both without a scroll.
Lycanroc: You can use a form scroll to swap between the Dawn, Dusk, and Day forms of Lycanroc.
Keep in mind all forms can be obtained by evolving a rockruff at a particular time of day.
Basculin: You can use a form scroll to swap between the Red and Blue forms of Basculin
Weavile: You can use a form scroll to swap between the Red crown and Blue Crown forms of Weavile
Vivillion: You can use a form scroll to swap between the 20 Vivillion forms.
You get the various forms of Vivillion naturally by evolving Spewpa in various biomes of MC.
Unown: You can use a form scroll to swap between the 28 Unown forms.
These two are in a small category of their own because they are necessary for Pokemaster challenges! Here is a link to all forms for the uninFORMed.
Non Legendary Pokemon Forms that work for entire Families of pokemon:
Flabebe Family: You can use a form scroll to swap between the Red, Orange, Yellow, Blue, and White forms of this family
Burmy Family: You can use a form scroll to swap between the Trash, Sandy, and Plant forms of this family
Deerling Family: You can use a form scroll to swap between the Winter, Spring, Summer, and Autumn forms of this family
Shellos Family: You can use a form scroll to swap between the West and East coast forms of this family.
All of the Alola Pokemon can be obtained (or reverted to Ordinary form) via a form scroll as well. These pokemon are:
Rattata, Raticate, Raichu, Sandshrew, Sandslash, Vulpix, Ninetales, Diglett, Dugtrio, Meowth, Persian, Geodude, Graveler, Golem, Grimer, Muk, Exeggutor, and Marowak.
Forms due to items in Pixelmon:
Reveal Glass: Transforms the Kami Trio (Thundurus, Landorus, and Tornadus) Between their Ordinary and Therian forms
Gracidea: Transforms Shaymin between its Sky and Ordinary Form (Only works during the day, and to use in battle, it must know wish)
Prison Bottle: Transforms Hoopa between its Bound and Unbound Form
DNA Splicers: Transforms Kyurem into its Black OR White Forms when Reshiram/Zekrom are beside him in the party. Merely right click him again to defuse them.
Meteorite: Transform Deoxys between its ATK, DEF, SPD, and Ordinary forms.
Multiple forms with Held items(Can't be changed with scrolls):
Red Orb: Transforms Groudon between its Origin and Ordinary Forms in battle (Held Item)
Blue Orb: Transforms Kyogre between its Origin and Ordinary Forms in battle (Held Item)
Griseous Orb: Transforms Giratina between its Origin and Altered form (Held Item)
Plates: Changes Arceus' type and its Judgement to that typing (Held Item)
Memories: Changes Silvally's type and its Multi Attack to that typing (Held Item).
To craft Memories, Place the Strongest TM of each type in the top middle, A magnet in the center, and a gem in the bottom middle, with aluminum ingots sandwiching the center column.
Rockstar, Belle, Popstar, Ph.D, and Libre Suits: Transforms Pikachu into the respective cos-player with its own special move.
To craft these outfits, Please watch this video:
I will update this guide when possible for future form releases.
Edit 1: Added Weavile to the list.