According to legend, a Chinese goat-herder noticed his flock had increased sexual activity after they ate the weed...Hence -"HORNY GOAT WEED"
It is actually Epimedium, also known as barrenwort, bishop's hat, fairy wings, rowdy lamb herb, randy beef grass or yin yang huo, and..horny goat weed.
It's a genus of flowering plants in the family Berberidaceae. There are about 50 species, the majority of which are endemic to China.
The active ingredient in Horny Goat Weed is Icariin which works by increasing levels of nitric oxide, which relax smooth muscle.