any pokemon in the undiscovered egg group is incapabe of breeding at all, not even with a ditto. The only exception to this is Manaphy, alegendary pokemon that can breed with any pokemon in the fairy or water 1 egg groups and ditto. Source:
None of the pokemon in this group (previously known as ‘No Eggs’) are able to breed, thus do not produce any eggs. The group mostly consists of legendary and baby pokemon. - taken from the link above.