Quite hard to believe. I have heard this but actually experienced this as the male that my female was in the tank with was a male Cobra Guppy and she was not anything like that, and 6-7 months later out popped Cobra Guppy babies and other looking babies like the other Guppy that was with her, and the females had been separated from males.
The Cobra looking ones were notable as there was only one male I had ever had like that and he had died.
The home the most of the Guppies came from were all red tail bluish silver body, other babies looked like all the rest of the group we had.