In almost all cases, the answer is no. All legendary Pokemon, except for one, are in the undiscovered breeding group, which means, like all members of this group, they are unable to breed. The one legendary able to breed, manaphy, produces a phione egg when left at the day care with a ditto. So, no, no legendary but one,even with ditto, can produce little copies of themselves, and the one that can produces a different Pokemon.
