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If you were looking for unstable hybrid Pokémon, see unstable hybrid.
An individual glitch Pokémon family contains all of the glitch Pokémon in Generation I under a certain Pokédex number. For example, 'M (00) and Missingno. are of the 000 family. The characteristics of families are different between Red/Blue and Yellow, and other versions, such as the Japanese Red/Green.
Glitch Pokémon that share the same Pokédex number (family) share the following traits:
Starting moves Base stats Typing Catch rate Base experience yield Menu sprite Palette attribute data
However, each glitch Pokémon has its own:
Index number Name Cry Level up learnset Evolution
This is because the ROM uses a data structure (see here) where certain data (including the first list of the above data except for palette attribute data, but not limited to) for all Pokédex numbers is defined, but not every trait of the Pokémon is defined in that structure.
Glitch Pokémon with the same family may also share the same front sprites, as is the case with 'M (00) and Red/Blue's normal Missingno. but not always. Other than the possibility of the sprite being partially random, if the Pokémon's index number is significantly different, it may have a different sprite due to the game using a different bank (see here for more information) for its three-byte pointer (note that only the bank changes, not the two-byte pointer).
Hybrid glitch PokémonA glitch Pokémon may share the same family as a valid Pokémon (#001-151). These are known as hybrid glitch Pokémon (not to be confused with unstable hybrids), and they share the attributes described above like typing and starting moves.
An example of a hybrid glitch Pokémon is 'Trainer'; a Magnemite hybrid. It also shares Magnemite's exact front sprite, unlike say the hybrid glitch Pokémon ゥL ゥM 4. Although ゥL ゥM 4 is a hybrid of Poliwrath, it does not share Poliwrath's sprite.
If a glitch Pokémon with an invalid family (#152-256, i.e. 000) is captured and its capture flag isn't set, it will become a Rhydon in the party or storage box after it is caught and its Pokédex entry is closed, due to the Rhydon glitch.
Due to all Pokémon in a glitch Pokémon family sharing the same Pokédex number; they also share the same 'encounter' and 'capture' glitch Pokédex flags, meaning that, for example, both 'M and Missingno. will add 128 to the sixth item if there are less than 128 items there due to them sharing the Pokédex number 000.
Why the variation existsThe Pokédex order table is stored in Red/Blue at 10:5024. These are the hardcoded families for the first 190 Pokémon by index number (as the Pokédex orders and index numbers are not the same). If the game attempts to access the family for Pokémon No. 191-256 (0), it will instead return code from whatever follows the table in the ROM.
In Pokémon Red and Blue, this covers 10:50E2 (InternalClockTradeAnim) and later trade related routines.
In Pokémon Yellow, the Pokédex order table is stored at 10:50B1. All glitch Pokémon families are taken from 10:516F (emotion_bubbles) onward, which is related to special Pikachu emotions. The addresses in Japanese Yellow revisions are different, but the glitch Pokémon families are still taken from the same source.