There are a number of mythical tales surrounding the origins of the Manx, such as that Noah cut off its tail with the door of the Ark as the rain began to fall. In actuality, Manx cats originated on the Isle of Man, off the coast of Great Britain, among a population of cats whose common ancestry sprang from the same roots as the British Shorthair. A spontaneous mutation occurred at some point several hundred years ago, which created kittens born without the vertebrae that form the tail of normal cats. With the passage of centuries and due to the isolation of the cats from outside breeding, the taillessness eventually became a common characteristic among the Isle of Man cats, because the mutated gene is a dominant trait.
The original Isle of Man Manx was a rangier cat than the standard used today, but the basics were there--deep flanks, long back legs, sturdy body. Through careful, deliberate breeding programs, the size of the cat has increased, and the short-backed, broad-chested and stocky cat that we see now became the desired type.