Perhaps we should really find out when metaphysics started as a discipline distinct from other disciplines, and by discipline I understand a study by man of things whatsoever at any time in any environment under any aspect whether real or unreal or purely fictional, possible or impossible or even endowed with contradictory features whatever, in objective reality or merely in the mind of man.
On that basis I would say that metaphysics acquired a specific distinction from all other disciplines when man notices that there is a difference between something and nothing, and seeks to draw up the features of something by which something is different from nothing.
At this point I would imagine ontology came about as a distinct study of mankind separate from metaphysics, because it is concerned with the kinds of something which something is already no longer nothing.
But in actual usage the two terms are used inter-changeably.
For example with atheist cosmologists, they want to understand nothing as something, but it is their ruse in order to tell common atheists who do not think deeply: that the universe came forth from nothing: so, no God is needed.
In that respect they are already into a wrong metaphysics and a wrong ontology, and man cannot draw that conclusion of their being wrong, unless he is cognizant of what is metaphysics and ontology and what is any other study of man distinct from metaphysics and ontology, and also the distinction between metaphysics and ontology.
Now, with theist cosmologists, the correct metaphysics is that there has always been something even before the varieties of something came to the notice of man; and the always something is God, the only self-existing something that is in effect the unique uncreated creator and operator of the created universe,in His fundamental relation to the universe that is studied by scientists or more in particular material scientists.
That is why atheist thinkers do not go for metaphysics and ontology and all kinds of philosophy, they just keep on insisting that material things are the only somethings to study for man.
But even the science of physics which is a material investigation of the ultimate particles and origin of the (material) universe is today cognizant of the fact that the material universe has a beginning 13.7 billion years ago.