Among the given options, the endangered species is the Blue sheep (Option 2). Endangered species refer to those animal or plant species that are at risk of extinction in the near future. Black buck (Option 1) is not endangered, as it is classified as "Least Concern" by the IUCN Red List. Gangetic dolphin (Option 3) is an endangered species and can be found in the Ganges and Brahmaputra river systems in India. Mithun (Option 4) is a domesticated animal and not an endangered species. It is important to protect and conserve endangered species as they play a vital role in maintaining the balance of ecosystems and contribute to biodiversity conservation.