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A Decamark or ???????? Pokemon can only be seen by cheating in Pokemon Ruby, Emerald and Sapphire. One way is to use a code to walk through barriers and walk through the trees above Littleroot town before getting a Pokemon from Professor Birch. When a wild Pokemon appears, you have no Pokemon in your party, so the Decamark is displayed as your Pokemon instead.

Appearance
The Decamark is a black circle with a question mark on it. It always has 0 health and has no moves. There is no name displayed. As soon as the opposing Pokemon attacks you, you "white out".