DoctorDraco
it kinda makes sense for there to be delta pokemon lorewise(kinda?) if an egg gets exposed to energy from a different type (or a other species parent) and doesn't die because of it but rather adapts to it you may get a variant. these variants aren't plentifull in nature because they get cast out by their herd when they are born and can't survive alone in the wild. those that do survive tend to grow up to become powerhouses.