Egg Groups

Mechanics

Every Pokémon belongs to one or two egg groups, which affect which Pokémon they can breed with. Specifically, the rules are:

Pokémon in the No Eggs Discovered egg group cannot breed at all.
A male Pokémon can breed with a female Pokémon as long as there is an egg group that both Pokémon belong to, and neither Pokémon belongs to the No Eggs Discovered egg group.
Any Pokémon which does not belong to the No Eggs Discovered egg group, and which is not Ditto itself, can breed with Ditto. This applies to genderless Pokémon as well as male and female Pokémon. (In fact, genderless Pokémon can only breed with Ditto, and so, despite the fact that they are assigned an egg group in the game's code, this assigned egg group actually has no significance in the gameplay.)