?????????? also called Decamark(s) for short by some people (because Deca means '10', so the literal meaning is '10 (question) marks'), refers to both many glitch Pokémon themselves, and the error trap used for them. The exceptions are ?, - (glitch Pokémon) and Bad Egg.
They occupy index numbers hex:0000 and everything up to hex:019D, but some ?????????? are too unstable to be worked with, because while some only freeze the game on the opponent's side, others freeze the game simply by hatching one or when hacking to have a Pokémon evolve into one.
Though the relevant glitch Pokémon will be referred to as "??????????" on the opponent's side, the same glitch Pokémon may have a glitch name on the nickname screen and summary screen, such as a shortened or unchanged version of a move name (e.g. "RBOLT", hex: 0201 instead of THUNDERBOLT) depending on its index number.
A ?????????? will have a sprite that is basically a white ? mark inside a black circle inside a white circle inside another black circle (see the bottom picture for what it is supposed to be). Certain ?????????? have messed up versions of this sprite though (see picture, left/right), which may be of a different color palette (possibly even to the extent the visible features of the intended ?????????? sprite cannot be seen), or may be flipped.
Many ?????????? can be obtained with the Glitzer Popping trick, or through the use of a wild Pokémon modifier code for a cheating device.
The hex:0000 variant does not have a glitched name on the nickname screen or summary screen. It is special, because it has its own Pokédex entry programmed into the game in Pokémon Ruby, Sapphire and Emerald and is considered as 'no Pokémon'. It will disappear from the party once captured, even though it is possible to force it in a party.
In Pokémon Ruby and Sapphire, all of its base stats are 0.
Its Pokédex entry in Ruby and Sapphire is:
"This is a newly discovered Pokémon. It is currently under investigation."
Its Pokédex entry in Emerald is:
"This is a newly discovered Pokémon. It is currently under investigation. No detailed information is available at this time."
Its species is the "Unknown" Pokémon. It is 0'00 tall and 0.0 lbs.
More research is needed for this article.
Reason given: Document more effects of it. Are the base stats the same across versions?