Pokémon Ranger
A select group of species cannot breed at all, which includes all Legendary Pokémon, all Ultra Beasts, most Mythical Pokémon, Nidorina and Nidoqueen (but not Nidoran♀), Unown, and all "baby" Pokémon. This is called the "Undiscovered" Egg Group. Although some Legendary Pokémon have genders, they still cannot breed. An unusual circumstance involves the relation of the Mythical Pokémon Manaphy and Phione; both can breed with Ditto to create an Egg which hatches into Phione, yet Phione does not evolve into Manaphy. Manaphy itself is obtained from an Egg from "Pokémon Ranger" games, but it will never breed to produce another Manaphy Egg.