MissingMons adds textures, models, animations and spawn for cobblemon that's curently not in the game yet but the catch is you guys get to vote for what i will be adding!
Effortless in-game skin customization and management—change, store, and customize your skins with SkinShuffle. Instant skin changes available on compatible servers.
CraftTweaker allows modpacks and servers to customize the game. With CraftTweaker you can change recipes, script events, add new commands and even change item properties!