Any Pokemon player should be aware of the various Glitch and slot Filler Pokemon that exist in almost every games coding.
the most famous, the one that brought the attention to the Slot Filler aspect of the game, MissingNo. and the Empty slot, 'M.
As attention grew, new ways of accessing OTHER filler slots and even trainer slots arose in generation 1, tricks using the Special Stats to encounter anything you wanted, including the 28 (?) MissingNo., empty, and Trainer slots that were encoded.
The reason for MissingNo. and these other slots? Hexadecimals!
Those funny little coding numbers that span 000 and 255. (expressed as 00-FF)
151 slots of those 255 are used for legitimate Pokemon, those near the end are used to hold Trainer Data, which allows for 30+ Empty/MissingNo./Hybrid slots.
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Generation 1 happened to be the only game where these slots were able to be accessed through "normal" means, but not the only one where they existed.
Good O'l #000 and 3 other Empty slot pokemon exist in Generation 2, and one for "EGG".
OBtaining these "glitches" in valid ways is possible, using a filler Pokemon in Pokemon yellow to trade to generation 2. (This trick also allows for Generation 2 pokemon to be traded in the Time Capsule)
Same reason for existence, filler, and egg data slots.
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Generation 3 was coded on a much larger scale than the others, with the spike in numbers of Pokemon, 255 Slots just didnt cut it anymore.
#000, the "Empty Slot" for having no Pokemon still exists, as it always does, but the hexadecimal code used in these games used 0000-FFFF instead of just 00-FF.
Using (dun dun dun) MATH! you can see that there is 65536 slots available for Pokemon.
Thats over 60,000 slots that contain the filler pokemon known as "??????????" (Though they start to have names like "CE PUNCH" or other snippets of attack names in that fashion)
For more info on these 60,000 fillers, search "DECAMARK"
The "Bad Egg" was also introduced in this generation to negate Hacking.
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Generation 4 used the same coding process as 3, but Decamarks were no longer the fillers.
After Arceus (Hex 01ED) Egg and Bad Egg are both programmed in. (Bad Egg and the Manaphy Egg share the same coded slot, so illegitimately trying to obtain a Manaphy egg will result in a Bad Egg)
All slots after that contain nothing. Cannot be encountered, caught, or fought with....
UNTIL! you reach Hex 8000.
Thats when the "Hybrids" start to appear in coding. Each one is statistically identical, but have a Sprite of a corresponding Pokemon that exists back in the 0000-01ED hexes.
0000=Blank Slot -- 8000=Hybrid of Blank Slot
0001=Bulbasaur -- 8001=Hybrid of Bulbasaur
01ED=Arceus -- 81ED= Hybrid of Arceus.
All the way through Bad Egg EDIT: Egg and Bad Egg Hybrids dont exist. After that, nothing once again.
(Hybrids cannot be caught, view stats, or battle off of an emulator.)
Hybrid Pokemon Image
Hybrid Gardevoir pokedex
Images taken on Emulator. The only way to obtain them on a DS is to use a Mining Museum code.
Through the Mining Museum code, Both the Egg slot and Bad Egg Slot can be obtained without being coded as an unhatched Egg, enabling both of them to be used in battle. (Their stats SUUUUUCK though, and they only know Splash)
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Sadly, Generation 5 only has one. Slot 000, due to better coding. (Bad Egg exists too as well)
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Thats my brief summary on the Slot Filler Pokemon.... Now... Discuss!
NOTE: Post has been edited. A few of my numbers were wrong.