Duck

With their sweet quacks and frequently vibrant plumage, who doesn't appreciate ducks? People who are aware of their dark secret, that's who. In the three percent of birds that have a penis, male ducks (drakes) are the most well-endowed. They are the finest endowed vertebrate species in terms of body size. For instance, the Argentinian lake duck's is four inches longer than its body. Duck penises are even more bizarrely corkscrewed, resembling pig tails with up to ten turns. Additionally, they are available in toothy, ridged, and even ribbed types, "like a selection of sex toys from a vending machine in a bizarre extraterrestrial bar."

You could argue that there is nothing wrong with that, but none of this developed "for her enjoyment." Actually, it's to keep her from escaping. In most duck species, there are more males than females, hence many drakes are unable to entice a mate. So they switch to gang rape, which is violent and frequently fatal.

Ducks have become so accustomed to this action that the penis has developed to hook into the vagina. Additionally, the vagina has developed its own defenses, much like a "sexual arms race." To catch undesired sperm, it could, for instance, spiral in the opposite direction of the penis or have "dead-end side pockets or cul-de-sacs." Some also have sharp twists or "teeth," evoking the folkloric vagina dentata.

Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Anseriformes
Superfamily: Anatoidea
Family: Anatidae