Well, yes and no, but mostly no. The things you named (dragons, flying spaghetti monster, etc.) are
extremely unlikely to exist, so I can say I believe beyond a
reasonable doubt that those things do not exist, a bit like how I can say I believe beyond a
reasonable doubt that existing religions do not have the pure and exact truth, although some principles of sikhism and what it implies about Hinduism, Islam and Christianism make sense to me. However, I cannot purely and simply prove that the flying spaghetti monster does not exist beyond
any doubt; it doesn't mean I have to believe in it. It's about probabilities, at that point.
As for the last line "do you really exist", I'll just give you a quote
Je pense, donc je suis.
- René Descartes, Discourse on the Method, 1637
Still true in 2020.