The penis is a sexual organ common to many species, particularly mammals and reptiles, and is specific to biologically typical males. In some species, such as humans, it has the extra function of allowing urination. Within human culture, the public display of this organ is generally taboo. In about 30% of human males, the skin towards the tip is removed in a procedure known as circumcision, which can be for religious or, it is argued, medical reasons.
In most mammals, possession of a penis is a reliable indication of gender. The female Spotted Hyena however has no vagina, and the clitoris is as large and as erectile as the male's penis - only the shape of the glans makes it possible to tell the sexes apart. The female urinates, mates and gives birth through this modified clitoris, during copulation, the opening widens to admit the male's penis.
Unsurprisingly, the blue whale has a rather large penis, at about ten feet long. In the 18th and 19th centuries, whalers were known to use the skin of the sperm whale penis as a one-piece waterproof apron. However, the common barnacle, has the longest penis in relation to the size of its body, - up to ten times the height of the barnacle when extended. 
The octopus uses an arm (the third on the right, called the hectocotylus) as its penis.
In Húsavík, the Icelandic Phallological Museum contains a collection of over one hundred penises and penile parts belonging to most of the land and sea mammals in Iceland, including twelve species of whale. The museum claims to have a legally-certified gift token for a future specimen belonging to Homo Sapiens.