Dark Pokemon generally use a dark colour palette that is directly related to their normal colour scheme. Like shiny pokemon their stats were elevated slightly above the norm in previous games making them slightly more attractive to trainers than Shiny Pokemon because of their 'cooler' appearance. Dark pokemon are completely exclusive to Pokemon Crater - Battle Arena and as such there are no particularly well known dark form pokemon. It is also unknown if dark pokemon would give off any kind of special animation when sent into battle