Water 1

Amphibians, turtles, seabirds and shoreline creatures: 94 of its 114 species are Water type. Manaphy and Phione are the only Mythical Pokémon that lay eggs, and a Manaphy egg never hatches another Manaphy: it hatches a Phione.

The Water 1 egg group has 114 species, 19 of which belong to this group alone. The groups it shares the most species with are Field, with 25, and Monster, with 15. Between them they can breed with 804 different species, Ditto included. All of them can breed with Ditto, and for the 2 genderless ones it is the only possible partner.