My guess is that these are throwbacks. Many years ago when I started in tropical fish we would breed guppies for upper swords, lower swords, and split tails.
The current lines of fancy guppies are all, to my knowledge, the result of breeding from these strains for the flowing tails and variety of colors now available. The ones that we have in our tanks today would not have a very good chance of surviving in the wild. The large tails would be a very big handicap in the wild.
It can happen that these fish, through breeding, will revert to a previous form. It is more likely to happen with crossbreeding of the various fancies but can happen at any time.