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.