Have you looked at Fire and Ice? It has the creatures. Maybe a bit more than you asked for. I code server side only, so can't help with the pistons, sorry
Well I'll look into the creatures thing, but all I really really want right now is the infinity star block, redstone things, and the colored blocks.
EDIT: So I understand that except it's not exactly the same as my horned beast idea. I basically a weirdo who wants harry potters unicorn in game is all, via a unique appearance of a different mob. I can get the other mobs from cavern, but they only spawn in the cavern dimension, so it wasn't really something I liked. Anyway, I'll continue to wait as I want this mod as to be an adventure mod. I've been wanting it for years, even working on an adventure modpack that goes with it. Going to become a pixi (tinkerbell), etc if you are into that thing, or go into outerspace, or go into depths of other dimensions browsing and looking through at least 200+ dimensions is what I aim. But I might add that mod w/ the other mods I find, so yeah. I really should learn to create my own game but I'm not good at coding, no teacher and etc. Oh well. I got to wait.