The Legend of Puto San Guelo:
Puto San Guelo is a demon who visits
teenaged boys and men
in the middle of the night, on the first new moon in the Spring, and tries to suck thier penis. According to the legend, if Puto San Guelo sucks your penis, you will become impotent (or your penis will shrink, or you will become gay -- there are several variations).
To protect yourself from Puto San Guelo, you need to cover your genitals in some way on that night, such as with a suitably sized cup or
athletic supporter ("ball cup").
When I was younger, my Latino freinds used to tell me of
the Legend of Puto San Guelo. Some would
actually wear
athletic supporters to bed on that night.