Let your mind go blank for a minute (for some of us, that is a perpetual state of mind). O.K., now I am going to throw a few pairs at you – Predator/Jotun, Silicon Entity/Unicorn, Slivine/Gorgon, Borg/Sea Creature. Forget for a moment that the Sci Fi creatures are scarier, and more deadly. Which seems more real to you? Remember, they are all equally fictitious. When we do start meeting other species, they might just as well look like reptiles and hunt as they might be really tall and prefer the cold.
Yet the beings from mythology do seem more real, somehow. When the Egyptians started forming their religion, they would take the animal associated with that quality, say intelligence, and mesh it with a human. That is why the Egyptian gods all look so weird. When the Greeks were overrun by mounted warriors, they didn’t realize that man and horse were separate – which is why centaurs became a part of their belief system. The point is that these figures of mythology aren’t just beings of religions we no longer believe in, they are a part of our collective psyches.
I have an admission to make. I’ve always thought that stories that employed minotaurs, pegasii, or werewolves were made by authors who had not been creative with that aspect of their stories. That connection that stories like Narnia and Tolkien make, or better yet the explanations to be found in the really good science fiction are deeper and more profound than anything a cool looking new species could manage.