Orion slave girls (also known as Orion animal women) were female Orions who were very popular "commodities" trafficked through the slavemarkets of the Orion Syndicate.
In 2154 some Enterprise crewmembers wound up as commodities themselves at a processing station on Verex III. When CaptainArcher expressed surprise that the Orions would sell their own kind, Arik Soong told him the females were "known for their extreme appetites, their innate skills". The Verex market's Orion slaver had even sold his last wife. (ENT: "Borderland")
Later that year the secret of the Orion slave girls' irresistiblity was discovered when the slave sisters Navaar, D'Nesh, and Maras were presented to Captain Archer, ostensibly to improve relations between Earth and the Orions. Their presence became a major distraction to the crew of the Enterprise due – it turned out – to the powerful pheromones which they were releasing. It was revealed that Orion females were not enslaved at all, but rather the enslavers, and the sisters were active agents in a plot to capture the Enterprise for their Syndicate. (ENT: "Bound")
In the 2250s, Captain Pike had some personal experience of Orion culture to weigh against Starfleet's 23rd century scientific insights. In 2254, his fantasy of doing business on the Orion colony at one point came to include Vina in the form of an "animal woman". One of his fantasized guests remarked that the natives of the colony were "funny" because they "actually like being taken advantage of". (TOS: "The Cage")
The Orion slave girls were first conceived in the story outline for "The Cage", as "sinuous green dancing girls" of Protos VI, who danced barefooted along with Vina, though she herself was not technically said to be one of them. (The Making of Star Trek, p. 58)
In the second revised final draft script of "The Cage" (which contained Captain James Winter rather than Captain Pike), the appearance of Vina as an Orion slave girl is described thus; "Wild! Green skin, glistening as if oiled. Her fingertips are long gleaming razor-edged scimitars, her hair not unattractive but suggesting a wild animal mane." After her eyes are also stated to be "wild," the script continues, "We feel she's larger than before, immensely strong [....] Now dancing wildly, animal beautiful [....] The green, animal Vina [...] seeming taller than Winter, perhaps even stronger."