Shadow Pokémon are one of the rare variant of Pokémon in Pokémon Vortex. Shadow Pokémon have immunity to status effects such as Sleep (This status is not yet added in game.)
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 Shadow Munchlax is a different colour. However, you cannot catch a Shadow evolved Pokémon, as all evolved Pokémon can only be caught in the Normal Variant. Shadow Legendary Pokémon, however, CAN be caught.