Mystic Pokémon are one of the rare variant of Pokémon in Pokémon Vortex. Mystic Pokémon have a chance to scare the defending Pokémon whenever they attack (scaring is another status effect).
They were made by staff of Pokemon Vortex. Hence they're not to be used on any sites other than pokemon-vortex.com and pokemonvortex.com.
Example:
This Mystic Munchlax is a different colour. However, you cannot catch a Mystic evolved Pokémon, as all evolved Pokémon can only be caught in the Normal Variant. Mystic Legendary Pokémon, however, CAN be caught.