The alligator story is one of many modern folktales (frequently with an ironic or horrific twist) that are retold as true, with embellishments to make them seem plausible. They appear in varying forms in different parts of the world and, like many stories, travel rapidly on the grapevine. Certain modern folktales have become classics.
In the 1930’s a curious craze swept across the United States. Americans started to buy baby alligators, which they reared as pets in their city apartments. But the novelty soon wore off, and as the pets grew too large and fierce, they were flushed down the toilet. As a result, some people claim that a race of giant sewage-feeding alligators - probably albino, since they live in permanent darkness - is thriving in the sewers of many a city...
More urban legends, debunked & well: