The myth about St Patrick chasing snakes out of Ireland is actually pretty funny. St Patrick was said to be enduring a forty day feast at the top of a hill, when snakes attacked him. He then chased these slithering snakes into the sea.
The story of St Patrick chasing snakes most likely never happened. Ireland is one of the very few countries in the world who doesn't have snakes. It stands alongside New Zealand and Iceland in this regard.
This human creation of snakes as villains has been replicated throughout literature and film. Snakes are not the only animal which humans apply a senseless and made up character of evil in an animal. Many animals throughout human history have been tainted with that brush, from rats, to owls, from foxes to wolves. All of these animals too have been created as the bad guys in the eyes of humans.
The St Patrick myth shows us how humans can create things in animals which do not exist. Whether is villain hood, or some other made up superstition.