I think the Pony of Shadows is a being of its own that affects the mind of ponies who are already in a state of extreme bitterness and hatred and turns them into different ponies entirely. It has to be willingly though, because Luna wouldn’t blame herself if it wasn’t. Maybe it lures them in with promises of power and revenge. Apple Jack said that it was remnants of Nightmare Moon, and maybe it is peices of an otherworldly spirit that once roamed the castle of the two sisters after it was separated from Luna (also explaining why it was haunted in CastleMania)