From Bulbapedia, the community-driven Pokémon encyclopedia.
Egg Groups are categories which determine which Pokémon are able to interbreed. Similarly to the types, a Pokémon may belong to either one or two Egg Groups.
Egg Groups are decided by biological traits, which overall serves to allow Pokémon which are biologically similar to breed. However, there are some oddities which have been noted within Egg Groups: for example, the Field Egg Group's standards are particularly broad, and many seemingly illogical pairings can be noted within it, such as Skitty and Wailord.
Egg Groups are preserved along evolutionary lines; Ralts, Kirlia, Gardevoir, and Gallade are all in the Amorphous group, while Pikachu and Raichu are in the Field and Fairy. Baby Pokémon are the only Pokémon besides Shedinja, Nidorina, and Nidoqueen that are not in the same group as their evolutionary relatives; Nidorina and Nidoqueen are both in the Undiscovered Group, while Shedinja is in the Mineral Group, despite having no gender, thus requires Ditto to breed.
Egg Groups have only been canonically mentioned in Pokémon Stadium 2, where they are a part of Earl's Pokémon Academy's extensive data banks, and Pokédex 3D, where they are mentioned along with various other information about the Pokémon in question. They are also mentioned in various strategy guides.
Many Pokémon are included in two different Egg Groups and can form what are known as breeding chains. These link many unrelated Pokémon together, allowing moves that would not normally be accessible to certain Pokémon to be legal, like Heal Bell on Blissey.
TMs are also passed down through breeding from the father. Any move in the father's current moveset that can be learned via TM, Egg move, or Move Tutor (in Crystal) will be passed down to the Pokémon hatching from the Egg.
This is a list of Egg Groups by their index number in the games:
- Monster Group: Pokémon in this group are saurian/kaijū-like in appearance and nature.
- Water 1 Group: Pokémon in this group are amphibious in nature.
- Bug Group: Pokémon in this group are insectoid (bug-like) in appearance.
- Flying Group: Pokémon in this group are avian (bird-like) in appearance.
- Field Group: The largest group, Pokémon here are terrestrial in nature. In Stadium 2, this Egg Group was known as "Ground".
- Fairy Group: Pokémon in this group are petite and considered very cute.
- Grass Group: Pokémon in this group are plant-like in appearance. In Stadium 2, this Egg Group was known as "Plant".
- Human-Like Group: Pokémon in this group are fully bipedal. In Stadium 2, this Egg Group was known as "Humanshape".
- Water 3 Group: Pokémon in this group resemble aquatic invertebrates.
- Mineral Group: Pokémon in this group are inorganic in nature.
- Amorphous Group: Pokémon in this group are amorphous, having no definite form. In Stadium 2 this Egg Group was known as "Indeterminate".
- Water 2 Group: Pokémon in this group are piscine (fish-like) in appearance.
- Ditto Group: As the name implies, Ditto is the only Pokémon in this group, and is capable of breeding with all others (regardless of gender) aside from those in the Undiscovered and the Ditto group.
- Dragon Group: Pokémon in this group are draconic in appearance.
- Undiscovered Group: Pokémon in this group are unable to breed with anything. In Stadium 2, this Egg Group was known as "No eggs".