The number of rhinos killed for their horns by poachers in South Africa dipped slightly in the first half of this year, but more than 500 were still slaughtered, the government announced Monday. South Africa is battling a scourge of rhino poaching fuelled by insatiable demand for their horn in Asia. Most of the demand emanates from China and Vietnam, where the horn is coveted as a traditional medicine, an aphrodisiac or as a status symbol.