Baculm
The os penis, a bone found in the penis of many placental mammals. It is absent in humans, but present in primates such as the gorilla and chimpanzee. The bone aids sexual intercourse by maintaining sufficient stiffness during sexual penetration. It is also suggested that certain shapes may assist in the removal of a rival’s sperm from the female vagina.
The female equivalent is the os clitoridis, a bone in the clitoris.

Walrus baculum, approximately 22″ (56cm) long

The size and shape of the baculum varies greatly between even closely related species and is often used as an aid in identification of species. But no-one knows why it is absent in humans, and relatively rudimentary in other great apes, compared with many other mammals.
The word is derived from a Latin for a cudgel, sceptre, staff or stick.