Of Eden, of course. Christians and the western world say the Serpent is Satan, duh. But it's worth noting that Genesis was not written by Christians. It was written by ancient Hebrews who did not have the concept of the devil that we do now.
In the story, this one creature is responsible for the existence of all suffering. It literally ruins everything. And it knowingly and sneakily causes this to happen. Why? What did the authors intend this creature to be? Where did such malicious intent come from?
And was this supposed to be a real snake, or a representation of something else? Voltaire once wrote:
It was so decidedly a real serpent, that all its species, which had before walked on their feet, were condemned to crawl on their bellies. No serpent, no animal of any kind, is called Satan, or Belzebub, or Devil, in the Pentateuch.