There is a comparatively new science of symbols called semiotics which partly explains why humans can actually create something new under the sun.
Animals don't use symbols or words in order to relate and react to their environment. They think differently to humans. They are at one with their environment (unlike us). Their mental mapping to the environment is one-to-one (unlike ours). But humans in the palaeolithic discovered symbolification (the clenched fist, the grunt, the mark on sand). This led eventually to words.
In some ways words screw us up and disassociate us from reality,from our environment. We have been able to manipulate symbols,manipulate words to create new ideas. Perhaps our brains are full of 'words' that just don't exist out there in the way that stone,river,fish do. We have devised words like God, honesty, status, alien, politics..and it can be argued that these things don't exist in the way that stone, river, fish do.
Things may not exist but they can still kill us. Perhaps the word 'Satan' is a good example. I understand it was once a harmless Hebrew word meaning 'enemy' or 'opponent' and carried no sinister connotations at all. But by the middle ages that word had really worked itself up. The personification of Satan has surely scared the life out of millions. Yes, this word has probably killed thousands. Real enough?
So the conclusion here is that a human mind can create a concept, a thought, a meme, a word that doesn't exist in the real world.. but can have an effect.
Memes can be like viruses. They can spread from mind to mind. In other words most of us could believe in something that just doesn't exist.
But I don't think it stops there. I think memes can lead to creation in the real world. Maybe a vampyre was once a meme,someone's thought creation. But now the idea has spread outwards, gathering power, until the word is lodged in the mind's of the majority.
Maybe people are seeing 'vampyres' in the media and saying 'Yes, I want that.' Maybe they are altering their patterns of behaviour. Maybe a tribe is developing, taking a separate path of development that will gradually separate them from homo sapiens. Maybe homo sanguivora is already out there... probably still a sub-species, but out there.