***CASTING NORMAL SPELLS WHILE TRANSFORMED WILL NOT WORK AND PROBABLY CAUSE YOU TO CRASH. I don't know any way around this other than forcing the transformation to remove all your gear and spells and lock the menus like werewolf/vampire do, which I felt wasn't worth it. Just unequip any normal spells you have before transforming.
*REQUIRES VERSION 1.8 OR HIGHER OF SKYRIM, does not require Dawnguard or Dragonborn.
Version 4 with playable ponies is up. The polymorph spells are in a chest along with the rest of them on the second floor of the Bannered Mare in Whiterun. They will allow you to freely transform to and from your regular character to the pony of your choice. Unlike vampire and werewolf transformations, your gear will remain equipped, though it will be invisible (except for your arrow quiver, which for whatever reason tends to get stuck in or near your knee). Your character menus will all work normally as well. Due to animation issues, there are still some things that are not possible, currently using regular magic while transformed will not work and cause it to crash; bows, sneaking, and shouts also will not work, though you can transform back freely in order to use those. Alternate versions of the basic pony summoning spells have been added that do work while transformed, as "call xxx" instead of "summon xxx", so you can still use those as well as any other lesser powers that you have. Each form has four unique attacks; left attack shoots a single projectile, right attack has an AOE (except for Applejack); upon transforming, your Z key will be set to that form's default special, which is unique to each, and usually results in things exploding (except for Applejack), and a master ability usually costing 300 mana can be found in the lesser power list. The ability name will start with the name of whatever pony you are transformed into to make them easier to find. All of them have an additional short duration speed buff as well called "Pony speed".
Current bugs include:
***REGULAR MAGIC DOES NOT WORK WHILE TRANSFORMED AND CAUSES CRASH
**Polymorph spells are currently not working for everybody; they get the abilities but not the graphic. I don't know why, and other polymorph mods have this issue as well. Player.setrace ponyracename (IE player.setrace ponyracederpy, player.setrance ponyracetwilight, etc.) may be used as well, though this is PERMANENT AND MAY RESET YOUR STATS (though it currently does not for me), so save your game before trying this. Player.setrace defaultrace (or just the name of whatever race you were before) should change you back. I am looking into how to fix this problem, but as I cannot replicate the issue myself and don't know what the cause is, I do not know if or when it will be resolved.**
Bows, sneak, and normal shouts do not work while transformed
Skill gains work for weapons but not armor work while transformed; you will gain destruction skill for most regular attacks while transformed
Occasional clipping through terrain
Must be in 3rd person to use regular attacks
Cannot ride horses while transformed (will fail or crash)
Event animations where the main character does custom animations, such as the execution in the beginning, will cause crash while transformed
More details on download pages
My Skyrim pony mod
Original Skyrim pony horse and voice pack mod by zdzichorowerzysta
Original pony models by KPshadowsquirrel
Jared Bangerter's Shapeshifting guide/script