First, humans are inventive, so they well could be made-up. Second, just because people only started systematically studying and cataloging fossils a couple hundred years ago does not mean no fossils were ever found before then. Ancient peoples could and did find dinosaur fossils and wonder what they had looked like in life.
What’s the problem with Noah’s having dinosaurs on the ark? I mean, if you lean hard enough into with God, all things are possible, no one’s going to be able to dissuade you, but you have to stick a whole lot of words that aren’t from God into the story to even kind-of justify it, and it winds up flying in the face of just about everything we know about animals. And boats. And storms. And the earth. And people. And history.
But hey, keep believing what you want, I guess.