Like ~UmbreonLover~ said, you can't breed a zygarde. The only
legendaries that you can breed with a ditto are those you get as
dex completion rewards. (They have "legendary" as their egg group
in the pokedex, for example deoxys and arceus. As pointed
out, lengendaries you cannot breed, like zygarde, have
"undiscovered" as their egg group.)