Egg Groups mark categories in which Pokémon can interbreed and is a feature introduced in Gen 2 as part of the overall breeding mechanics. Comparable to Pokémon types, every Pokémon belongs at least to a single Egg Group, if not two.
These Egg Groups are mostly based on real-life taxonomies (meaning the classification based on biological traits and shared characteristics), allowing Pokémon to breed that are somewhat similar in appearance and abilities. However, this reasoning does not apply to all Egg Groups as some Egg Groups contain biological anomalies, supernatural and mythical creatures making “scientific” explanations for their classifications complicated. These inconsistencies are highlighted by seemingly illogical pairings like Wailord and Skitty – a community favorite.
It is to note, that most Pokémon retain their Egg Groups throughout their evolutionary lines. However, some Pokémon are known to either change their Egg Groups during the maturity process (baby Pokémon) or lose the ability to reproduce. Losing the ability to breed is usually accompanied by switching to the Undiscovered Egg Group (such as Nidorina and Nidoqueen).
Chain breeding is a method that utilizes the secondary Egg Groups of many Pokémon to pass down moves to their successors that they usually would not be able to learn on their own. Additionally, this method also allows the offspring to inherit any attack from the father that is flagged as an egg move, move tutor move, or was learned through the usage of TMs.
If you want to know more about breeding Pokémon in general, check out our breeding section.