The animal with the largest penis relative to body size is a barnacle.
That's an interesting bit of penis trivia
(I think)
edit :
And some more penis trivia, some species of flatworms engage in penis fencing. These species are hermaphroditic so when they engage in sex, either one of them can become inseminated and thus become the mother while the other one becomes the father. None of them actually want to be the mother, so to figure it out, they fence with their penises and try to puncture eachother's skin to deliver the sperm load.
You can see their white, pointy double-schlongs in this picture
