Shadow Pokémon are one of the rare variants of Pokémon in Pokémon Vortex. Shadow Pokémon have immunity to status effects such as Sleep (this effect is not yet added in the game.)
They were made by the staff of Pokémon Vortex. Hence, they are not to be used on any site other than pokemon-vortex.com. Example:
This Shadow Munchlax is a different color. 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.