Each Transformer was originally a monster. And so, when any of them would transform, they'd end up retaining features of their monsters.
And that would add some ferocious nature to their spirit power. Not to mention when they'd ask humans to sculpt statues for them, these statues would usually have features of their original monstrous nature.
That was why William recognised them on the spot. The statue that appeared got a face that was similar to a wolf, with weird protruding jaw and canines of wolves instead of human teeth.
As for why they were scary despite being just spirits entrapped and imprisoned inside statues and rocks, William already knew the reason behind this.
They'd keep their memories and knowledge and would use these to climb over the ladder of power in the human world. And that made them the most annoying type of enemies anyone would have.
They weren't dangerous in themselves, but they were hard to deal with thanks to armies of humans following them.
