Prolog-Before She Disappeared

Most Legendaries can have Heirs, but not all. They chose their own heirs. It's more like, "HEY I WANT YOUR DAUGHTER AS MY HEIR SO NOW SHES MINE!" No one really questions it, and plus the majority of Heirs were originally orphans.
Mythical Pokémon can also have Heirs, though they mainly chose not to.
UBs, or Ultra Beasts, that's just shaky territory.