Localized into all languages the base game supports! Screenshots in English though.

You can convert any sprite sheet to work with this mod, but hundreds are compatible out of the box due to backwards compatibility with all Adopt 'N Skin mods.

Does:
-Randomizes the animal appearance you get from Marnie from any of your installed variant packs.
-Works in multiplayer and singleplayer, even with your horses. [Let me know if issues.]
-Works with mod added pets and livestock. [If issues, let me know which mod.]
-Works with old Adopt 'N Skin packs.
-Doesn't replace any vanilla content, only adds to it.
-Change your pet, farm animal, or horse sprite appearance by clicking it while holding the color wand item. This opens a preview menu for that species. You may also rename your pet on this screen:

Install this mod the way you would any other SMAPI mod, in your mods folder. Now open the folder and you should see PUT YOUR SKINS HERE. Literally paste all of your variants into this one folder. If any share the same name from different mod authors, just rename their numbers and that's it. Hooray, you're done!

Craft this item in game. Put it on your hotbar. Approach an animal. Left click the animal. Use the menu to change its color and name.

To make a pack just do this:
1) Unpack animals with StardewXnbHack. Open 'Animals'
2) Look at the name of the animal.
3) Create your new spritesheet and name it the same thing as the animal it's replacing, but with _1 at the end. Then _2 for the next variant of that species, and so on. BrownCow_1 BrownCow_2 BabyBrownCow_1 etc
4) If it's a sheep, make a sheared version. Horse's are just Horse_1 Horse_2, etc.

BabySheep_1 Sheep_1 ShearedSheep_1 [Three separate sprite sheets, just like vanilla]
BabySheep_2 Sheep_2 ShearedSheep_2
Horse_1 [One sprite sheet, just like vanilla]
Horse_2

Simple Animal Variants