Foreskin

In mammals, the foreskin or prepuce is the retractable double-layered fold of skin and mucous membrane that covers the head of the penis, (the glans penis). It serves as a sheath to protect the glans. All mammals other than the monotremes http://encarta.msn.com/encyclopedia_761553537_2/Reproductive_System.html (the platypus and the echidna) have foreskins. This includes marsupials like the koala http://www.uq.edu.au/news/?article=2193 and marine mammals such as the whale and the dolphin http://www.doc.govt.nz/Publications/004~Science-and-Research/DOC-Science-Internal-Series/PDF/DSIS104.pdf.

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In females, the clitoral hood (see that article), is an analogous structure.

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Prepuce: The prepuce is a retractable piece of skin which covers part of the genitals of primates and other mammals....

Penis: The penis (plural penises or penes) or phallus (plural phalli) is the external male copulatory organ of some animals, and, in mammals, the external male organ of urination....

Glans penis: The glans penis is the sensitive erectile tip of the penis. It is wholly or partially covered by the foreskin, except when the foreskin is retracted, such as during sexual intercourse while the penis is erect....