Your subconscious is capable of breaking something down and recreating it; taking different pieces of what you've seen and experienced to create something new. My example would be J.K Rowling's Quidditch. She took flying and basketball, two things that already existed, and made it something entirely new.
I think I understand what you're talking about, but the truth is, everything crazy and outlandish that we already have and will have are more like inspirations from what we've already seen. Walruses and manatees are believed to be the first "mermaids," simple sharks and squids were blown all out of proportion to inspire all kinds of sea monsters, komodo dragons were just that, DRAGONS that breathed fire and caused mayhem, etc.
So while these inspirations existed, many people let their imaginations just go crazy and gave them wings, fire, tails, magical powers, etc.