Unique works like this:
The only POSSIBLE way you can get one is if you played one. Neither fate eggs, mutations, twin universe, nor fractal can target it.
A unique creature would be like a deadly assassin. You can't just make a mutation that happens to turn into a deadly assassin. As cool as that would be, it doesn't work that way. You have to train deadly assassins, and there's only so many in the world. (in our case, 12). This is different from other cards, where it's a species, and if one just wakes up from inside an egg it's intelligent enough to go whack that guy.