Now for a long time there have only been 5 species of rhino, two in Africa (White and Black) and three in Asia (Indian, Javan and Sumatran). The Javan rhino has for a while been the most critically endangered with only 60 individuals across two locations. In fact the Vietnamese Javan Rhino subspecies miraculously survived the idiotic war there but now appears to be less than 5 individuals with speculation they are only females.
However the Javan rhino may now lose its mantle as the rarest rhino species. A recent report has suggested that the Northern White Rhino (Ceratotherium Simum Cottoni) is not a subspecies of the White Rhino but a district sixth species. This has profound results. No longer is there a chance of saving the Northern White Rhino by cross breeding with the plentiful Southern sub species. Moreover no longer is it merely a sub species that is in danger (many rhino subspecies have become extinct already), but an actual species that has less than 20 individuals remaining.
Another report located here explains how Northern White Rhinos have been shipped from a zoo to Kenya in a last ditch attempt to save them.