Things that have to do with the nature of being are ontological, like your ontological question about the Loch Ness Monster — do we have to have proof in order for something to exist?
The study of being, is called ontology, so an ontological argument might investigate whether God — or people, for that matter — really exist. Are we real or just an imagined phenomenon? How do we know we aren't just characters in our creator's mind? These are heavy questions — philosophers including Aristotle, Parmenides, Descartes, Hume, Kant have all weighed in on the ontological question of “what can we be certain exists?”