Can Ditto And Arceus Have A Egg?

Has nothing to do with his gender.
He cannot breed because he is a legendary pokemon. I.e.: He is somewhere in the storyline and only one is obtainable in the game.
Manaphy is the only legendary that can breed. It will create the pokemon Phione, but he wont evolve into manaphy

No because Arceus does not have a gender

No not with any legendary pokemon p.s manaphy is not a legendary