As with nearly any aspect of the body involved in sexual or excretory functions, the penis is the subject of many slang words and euphemisms for it, a particularly common and enduring one being "cock".

[6] The external genital organs appeared in the Devonian, about 410 million years ago, when tetrapods began to abandon the aquatic environment.

[8] In fact, the necessity to overcome the absence of a liquid phase in which to release the gametes was achieved through the transition to internal fertilization.

Among amniotes, the development of an erectile penis occurred independently for mammals, squamates (lizards and snakes), testudines (turtles), and archosaurs (crocodiles and birds).

[9] The penis is an intromittent organ used to transfer sperm into the female genital tract (i.e., vagina or cloaca) for potential fertilization.

During ejaculation, a series of muscular contractions delivers semen, containing male gametes known as sperm cells or spermatozoa, from the penis.

A bird penis is different in structure from mammal penises, being an erectile expansion of the cloacal wall (in ducks) and being erected by lymph, not blood.

In placentals, the urethra, which is connected to the vasa deferentia, travels through and exits the penis, thus both urine and semen are expelled from this organ.

[22][23] During copulation, the spotted hyena inserts his penis through the female's pseudo-penis instead of directly through the vagina, which is blocked by the false scrotum.

Species of this genus have sex-reversed genitalia: females have penis-like organs called gynosomes that are inserted into vagina-like openings of males during mating.

[38] Meanwhile, critics have argued that it does not fit the intromittent organ definition of "a structure that enters the female genital tract and deposits sperm".