What is a Decamark?
??????????, also known as Decamark (being derived from Deca + question mark), is a glitch Pokemon seen in various Generation III games.
There are as many as 65,097 of them because they occupy the rest of the space inside of the Pokedex that isn't taken up by standard Pokemon, alternate forms of Pokemon, and so on. It is a placeholder entity called forward when the game tries to load invalid Pokemon data, preventing the game from crashing horribly. Thanks Decamark.
A majority of the Decamark forms are very unstable, but some can be used for performing glitches, most notably the Pomeg Berry glitch. Some Decamarks can be used for Arbitrary Code Execution as well! I won't be going over any of that stuff here though, but I might put up a page in the future about my glitch endeavors once I beat Pokemon Emerald.
Level Up Method
So apparently if you place a Pokemon in the Daycare before executing a certain Gameshark code in FRLG, it can produce an egg that hatches into a Decamark. This Decamark comes out at Level 0 and SUPPOSEDLY takes 1 MILLION EXP TO LEVEL UP? I know you can mitigate this with a Rare Candy but jeez man. 1 MILLION EXP! Who even has the time for that anyway?
Interestingly, if you trade this Decamark to Pokemon Emerald and THEN put it in the Daycare, it'll level up to 100 super quickly! You can teach it any of the HM moves too which makes it kinda like a regular Pokemon if you think about it? I honestly might try this for myself on my own copy of Emerald because I've always wanted a Decamark of my own. :D Don't try and give it any Rare Candies after this though because it'll start forcing you to teach it Ice Punch over and over and that's going to suck a lot. OH and don't look at its summary either because it'll crash your game.
Walk through Walls
If you use a Walk through Walls cheat to go out in the grass before getting a Pokemon in the early game of R/S/E and FRLG, your player character will send out a Decamark! Your character will then proceed to white out and appear in a Pokemon center, to which dozens upon dozens of Pokeballs will appear on your screen and crash your game. C: There are technical reasons behind this occurance that I don't feel like talking about. You can find a lot of this on Bulbapedia anyway.
Bulbasaur
Decamark's party sprite can sometimes appear as a Gold colored Bulbasaur ingame. I believe the reasoning for its gold color is because of a placeholder pallete being called in place of a nonexistent one, but why it defaults to Bulbasaur is beyond me. Decamark (some versions of it, at least) also possess the same move learnset as Bulbasaur in Gen III. ADDITIONALLY, some Deca varients use Bulbasaur's footprint when viewing their Pokedex entry ingame! Truly riveting, I know...
Hex:0000
So unlike other Decamark varients, Hex:0000 (with 0000 being its hexadecimal identifier internally) is a bit weird. It seems to be one of, if not the most NORMAL Decamark ever. When you catch it, it just disappears from your party, it doesn't do anything weird to your game (that I know of) AND it even has its own Pokedex entry! Check it out.
Aint that something? It's certainly a special little fella.
Mystery Dungeon Decamark
Decamark also makes an appearance in Pokemon Mystery Dungeon!
In the English version its known as ??????????, but in the Japanese version its known as 何者か ! When translated, you get "Someone." Very creative name...
It's a Normal type with the Overgrown ability, and its recruit chance is -999. This thing HATES being recruited I guess.
It strangely has its own dialogue too? I'll leave this screenshot here, which I took from TCRF.
I too say "Something..." when I'm out of words to say. Truly inspirational stuff, Decamark!