Dark Pokemon

Some people believe dark Pokemon are created from the hate that swells inside a Pokemon making it impossible for it to change back unless it goes through a life changing event. But, the true way to make a Pokemon become a Dark Pokemon is to use a Dark Ball. Dark Ball's are specific Poke Balls said to be created by the Iron-Masked Marauder. They usually don't show up in normal Pokemon games, but they do sometimes show up in Pokémon episodes/movies. It is a unique Poke Ball which first appeared in Celebi: Voice of the Forest. If anyone's seen the movie, they would know that a Dark Ball is known to augment a Pokemon's abilities, allowing them to surpass the limits of the original Pokemon's powers by bringing a Pokemon to the highest level. It completely changed the innocent and sweet flying Celebi to a monster destroying her own home, the forest. Of course, it took Ash's bravery just to save and return Celebi back to normal. Another thing the Dark Ball does is it is also able to bring under control Pokemon that already belong to a Trainer, as well as rendering it utterly void of its free will. So practically if you send out your Pokemon against a trainer using Dark Balls, then your most likely going to lose your own Pokemon if the trainer takes interest in it. It is also noted that a particularly strong-willed Pokemon can break free from the control of a Dark Ball, but the repercussions can be equally life threatening for the Pokemon. Dark Balls are dangerous, yet fun to use on Pokemon. It is also true at the fact that Pokemon under the influence of Dark Balls also bear similarities to Shadow Pokemon. A cool factor about Dark Balls is that Dark Balls are also shown to alter the target Pokemon's color; in which their skin tone is darkened.

Now here are a few pictures of some Dark Pokémon.
Dark Tyranitar Normal Tyranitar