A Legendary mutant and its Elite versions can only be bred by cross-breeding the same mutant with any another mutant which shares at least one gene type. Therefore, you must acquire one of them through the ways other than cross-breeding in order to proceed to the next step for breed its Elite versions.
Some Legendary mutants are excluded permanently from the breeding rule said above since the change at May 26th 2016, which allows Psy Captains to breeding them just normally like other breedable mutants, regardless of using the same or specific parent(s).
The following list indicates all breedable Legendary mutants:
Some Legendary mutants that aren't listed above can partially be bred during the limited Breeding Event, and some temporary breeding rules may require specific parent(s) to breed them.