Odd penises aren't unique in the animal world – and to the animals sporting them, of course, they're not odd at all. Snakes have two half-penises that join when it's time to mate [source: Medical Dictionary]. Sea-slug penises are disposable [source: Kaplan]. Cat penises are barbed [source: Dell'Amore].
But the echidna, often called a "spiny anteater," tops them all. The Australia-area native sports a single shaft with four heads, each one capable of ejaculation. Only two of the heads function at any given time, though, which makes sense since the female echidna has "only" two vaginas. During copulation, one side of the penis is inactive, its two heads retracted, while the other side is ready to fertilize the female's eggs. After ejaculation, the male echidna activates the other side and repeats the process [source: Cooke].
No one is entirely sure why there are so many darned heads, but experts hypothesize it has something to do with mating competition. Female echidnas might have sex with 10 or more males at mating time, so having four ejaculation sources probably increases the odds of any one echidna passing along his genes [source: Cooke].