?????????? 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.
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.
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 glitch Pokémon species name and Pokémon data (Types, Exp Curve, Learned Mosevet, Base Stats,...) are read on ROM data depending on its index number.
Most of the Species Name/Pokémon Data of the first thousand Glitch Pokémon are common between different language versions of Ruby & Sapphire / Emerald / FireRed & LeafGreen.
However, aside from the Species name of the very first glitch Pokémon that are based off moves names, most of a Glitch Pokémon Data is completely different between different versions of Generation III games (differences between an English Ruby and an English Sapphire, differences between an English Ruby and an English Emerald, differences between an English Ruby and a Spanish Ruby), leading to hundreds of thousands of different Glitch Pokémon across 6x5=30 Generation III Games.
In every Generation III game, when a NPC such as Slateport Pokemon Fan Club Journalist or Island Two Old Woman reads a Glitch Pokémon species name, the whole species name is read, which overwrites a string of RAM data in the process.
Every NPC that read a species name except Slateport Pokémon Fan Club Journalist will freeze the game when reading a species name who contains more than 100-200 characters.
Making a glitch Pokémon gain levels can freeze the game (supposed to either be because of an evolution attempt, loading the glitch pokémon species name, trying to learn too many moves at once, trying to learn a glitch move, or not being able to parse with the experience curve).
In Emerald and Fire Red & Leaf Green, a glitch Pokémon species name is fully read (and fully written in RAM) when the player goes over the Glitch Pokémon in the PC. This RAM overwriting messes with graphical data related to PC graphics. Changing PC Boxes and selecting Pokémon can quickly and randomly crash the game, while closing the PC will clear off all the graphical issues.
If the species name is long enough, the overwriting can mess up with the Pokémon Selection mechanic and allow for Cloning and Reverse Cloning. Species name even longer can alter the music as well as the overworld background, even though these effects are not retained by saving and soft-resetting.
The cloning procedure also generates a PC Pokémon with data taken from the Glitch Pokémon's species name. If the PC Pokémon data is empty enough, an Invisible Bad Egg (a corrupted hex:0000 ) may be generated. An Invisible Bad Egg can be used to deposit every Party Pokémon that was alive with a technique called Decaswitch.
The inverse cloning procedure from Glitch Pokémon species name is also the only way to discard Bad Eggs obtained in FireRed and LeafGreen.
Having a Glitch Pokémon that allows for cloning and inverse cloning in FireRed and LeafGreen is enough to make these versions autonomous regarding Pomeg Glitch setup and Pomeg Glitch Data Corruption (no more trades from Emerald are needed).
Some Glitch Pokémon species name can be used to change the color of certain graphical entities of the PC Screen depending on the currently selected Pokémon : https://www.youtube.com/watch?v=kOXh7bZRlMo
In Fire Red and Leaf Green, a Glitch Pokémon species name is always replaced by ?????????? in the summary, and its summary sprite is always the ? white mark inside circles.
The PC sprite of a Glitch Pokémon will always be a copy of the sprite of a Pokémon in the same box, but with another color palette. However, the sprites of the Glitch Pokémon's Glitch Types can cause heavy graphical glitches in the summary screen.
Some Glitch Type sprites can turn some PC Pokémon into Bad Eggs if the player goes to other summaries, allow to see the summary of empty slots, allow to see some summaries of PC Pokémon that are in another box, or freeze the game. As a result, the summary of a slight portion of Glitch Pokémon cannot be opened safelt/without crashing in Fire Red & Leaf Green.
Glitch Pokémon in Fire Red & Leaf Green seem to be hatchable with no major issue as their sprite is stable and as their species name is not written in RAM.
In Emerald, only the 10 first characters of a Glitch Pokémon are read in the summary page.
A Glitch Pokémon Sprite can be the ? white mark inside circles, a variant with a different color palette, a block of glitchy mess, or a glitched sprite that crashes the game.
The Pokémon's second sprite and sprite animation is supposed to be the cause behind the important number of Glitch Pokémon that crash the game when their summary is seen.
Glitch type sprites can also freeze/crash the game. There seems to be more glitch types that crash the game in Emerald than in FireRed & LeafGreen.
Most of the Glitch Pokémon in Emerald crash the game when their summary page is opened, or when they hatch from an Egg. Thus, a double corruption is mandatory to obtain them in order to not go through a hatching sequence.
In Ruby and Sapphire, the whole species name is read when viewing a glitch Pokémon summary. The glitch Pokémon sprite is always the ? white mark inside circles.
It seems that only a handful of Glitch Type Sprites can freeze the game, even though viewing the summary screen of a Glitch Pokémon causes a string of bytes in RAM to be overwritten by the species name. This RAM overwriting can be used to remove the Safari Mode while inside Safari Zone, even though a large part of the story flags and bag contents becomes corrupted (https://www.youtube.com/watch?v=yD7UCBv0ozA ).
All of the ?????????? can be obtained with the Pomeg corruption glitch by performing a double-corruption on certain PC Pokémon, or through the use of a wild Pokémon modifier code for a cheating device.
It is also possible to see the Pokédex entry of a Glitch Pokémon by trading data with a save file who possesses that glitch Pokémon in its party and by stealing it from the secret base trainer that appeared after the data exchange.
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. Can a Glitch Pokémon evolve if his sprite and name are "stable" enough ? Are there many glitch cries available for Glitch Pokémon ? List Glitch Type Sprites that could cause interesting effects. Study the causes and effects of the corruption of the PC Pokémon Selection in Fr/Lg. Test again the effect of Pokédex entries of Glitch Pokémon.