Somebody asked me earlier today why a penis is also called a cock. I assumed that it had something to do with the rooster (the male chicken) being called a cock. While this is true, there's a little bit more to tell. Here's a brief investigation into the matter.
According to Etymonline:
Slang sense of "penis" is attested since 1610s (but cf. pillicock "penis," from c.1300);