A fear of snakes is among the oldest of all of humankind's basic fears, so it is likely that this urban legend grew out of the ancient warnings of primitive people to be cautious about the places they selected to relieve themselves when obeying a call of nature. As civilization progressed, the outdoor toilet was a place that often harbored snakes that would scare or bite a person, which would possibly lead to death. With centuries of apprehension about snakes it seems a natural progression to bring such primitive fear into man's modern plumbing and toilet facilities.
The story: Perhaps the most common version of this urban legend has someone with an enormous pet python moving into an apartment building. In order not to alarm his neighbors, the individual keeps the nature of his pet a secret from everyone. (Sometimes the story states that he or she is with a traveling circus and is only staying in the apartment for a few nights.)
One day, the snake fancier carelessly leaves the toilet lid up in his bathroom, and the big serpent wiggles its way into the bowl, enters the drain pipe, and emerges in the toilet of the next door apartment. The startled individual is horrified to see a monstrous snake suddenly rear its head from the toilet bowl, and as he runs from the bathroom, the mammoth python begins to slither its way into his apartment. This account usually ends with the frightened apartment dweller calling the zoo or the animal control center and a crew of men arriving to wrestle the monster python of 15 to 20 feet out of the building.
Another popular version of the story has a big snake somehow making its way into the plumbing system of an apartment house and moving through the pipes until it comes up for air in someone's toilet just as the person is about to use it. In some legends, the victim either dies of a heart attack or is bitten to death by the deadly reptile.