Unless I'm mistaken, there is a species of spider that can grow to be about as tall as a pony, but is it not native to the deserts of Africa and Mesopotamia?
Originality is essential in writing a good book. I'm not saying that you have to look for the definitive original idea; you could spend your whole life looking for that and never find it. As Ecclesiastes says, there are no new ideas under the sun; everything has been thought of, even extra-terrestrials! People have been talking about aliens ever since people with minds walked upon the earth. What makes a story original is incorporating an idea into your own personal experience and character to produce an original product. Even The Lord of the Rings was based off of old ideas; it was Tolkien's utilization of new viewpoints on old ideas that made The Lord of the Rings not only an original, but a definitive foundation to the fantasy genre as we know it today.
Wow, I get to talking about writing and I can rant with the best of 'em! Our English II teacher would be so proud...