Från Wikipedia magret hänvisar till en specifik ras ( Mulard , för att inte vara förvirrad med Mallard):
Magret refers specifically to the breast of a mulard or Barbary duck that has been force fed to produce foie gras.
Från dartagnan.com :
Sometimes called “duck steak,” the magret (breast) of the Moulard duck is known for its rich flavor and dark red meat. Try it pan seared and find out why the French swear by the duck.