From Bulbapedia, the community-driven Pokémon encyclopedia.
A glitch Pokémon family is a group of glitch Pokémon (and possibly one non-glitch Pokémon) that share a Pokédex number, base stats, color palette, starting moves, and types. Families with a Pokédex number of 1-151 contain the non-glitch Pokémon with the same Pokédex number. Glitch Pokémon families are only found in Red, Blue, and Yellow Versions. Families in Pokémon Red and Blue are completely separate from families in Yellow. Pokémon that share a family with a non-glitch Pokémon are sometimes known as hybrid glitch Pokémon (not to be confused with unstable hybrid Pokémon).
In Generation I
The game mechanic "family" is used to define the characteristic of any species with a given Pokédex number in Generation I. Any glitch Pokémon with the same family as a legitimate species will also share its base stats, color palette, starting moves, and types. For example, Q ◣ is part of family 121, so shares these traits with Starmie; however, due to the way that glitch Pokémon change identity when traded from Pokémon Red or Blue to Yellow, and vice versa, Q ◣ will become 'M (FF) in Red or Blue, which is part of family 006, meaning that it shares these traits with Charizard.
Generation I glitch Pokémon may also be in a family which does not correspond to a real Pokémon, such as 000. If so, the base stats and color palettes of those glitch Pokémon are the same, but only within the same version (except between the same localizations of Red and Blue). For example, the glitch Pokémon 'M (00) from Red and Blue (but not in Yellow, as it is 3TrainerPoké $) is in the family 000; the glitch Pokémon g g in Yellow is also in the family 000; however, these two Pokémon do not share the traits that Pokémon in a family would share due to being in different versions. The same situation occurs between different languages.
List of glitch Pokémon by family
Similar concepts in other generations
In Generation II
The four glitch Pokémon named ????? all share several properties, in a way somewhat similar to families. However, they all have different Pokédex numbers, and do not share most of the properties that Generation I families do.
In Generation IV
In Generation IV, cheating-exclusive Pokémon, known as Generation IV hybrids, can be considered both to all be in a family, and to be in a family with their non-glitch counterparts. Their Pokédex number is the negative of a non-glitch Pokémon: ?68 to ?61 (i.e. 32,768 to 33,261, or -1 to -493 according to the signed representation of those numbers). All Generation IV hybrids use a 'floating version' of the original Pokémon sprite—the sprite which is used for Magnet Rise.