The term "musk deer" refers to any of seven species of proto-deer that comprise Moschus, the only genus of the family Moschidae, and are also known by the technical term Moschids. Musk deer are more primitive than most other deer in a variety of ways, most notably in their lack of antlers, and thus are not considered "true deer."
Musk deer are named after the fact that the males of all seven species have a musk gland - a sack located close to their genitals that is highly sought after to use the musk contained therein in perfumes and for medicinal uses in traditional cultures. The word "musk" derives from the Sanskrit word muṣká, meaning "testicle," which makes sense because its properties as a perfume and medicine were first discovered in India.
Musk deer once ranged widely across Asia and Europe, but their numbers have been greatly reduced due to hunting them for their musk glands and habitat loss. Today they occupy a tiny fraction of their former range and 6 out of 7 species are endangered species, with the seventh listed as "vulnerable."
The threat from poaching continues, and despite the invention of synthetic musk, real musk remains one of the most sought-after substances on Earth. Today a single musk gland can fetch on the order of US$5000 on the black market.